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{{Era|Flood|FFW|HCW}}

{{Cleanup}}
{{other uses}}
{{Covenant Species Infobox
|name=''Inferi redivivus'' (Flood)
|image=[[File:Flood Tank Form.png|250px]]
|homeworld=Unknown origin, Extragalactic (first Forerunner contact at G617g)
|height=Variable (depending on host)
|weight=Variable (depending on host)
|skincolor=Variable: green, black, tan (depending on host), however, many hosts mutate to brown and green
|distinctions=Significant physiological and psychological mutations. They can also acquire their host's memories.
|equipment=Variable, stolen from their hosts
|types=*[[Gravemind]]
*[[Proto-Gravemind]]
*[[Flood Juggernaut|Juggernaut Form]]
*[[Pure Form]]
**[[Flood Stalker Form|Stalker Form]]
**[[Flood Ranged Form|Ranged Form]]
**[[Flood Tank Form|Tank Form]]
**[[Flood Infector Form|Infector Form]] {{C|cut}}
**[[Flood Transport Form|Transport Form]] {{C|cut}}
**[[Flood Stealth Form|Stealth Form]] {{C|cut}}
*[[Flood Combat Form|Combat Form]]
**[[Flood Combat Form#Human Form|Human Combat Form]]
**[[Flood Combat Form#Sangheili (Elite) Form|Sangheili Combat Form]]
**[[Flood Combat Form#Jiralhanae (Brute) Form|Jiralhanae Combat Form]]
**[[Flood Combat Form#Unggoy and Kig-yar Combat Forms|Kig-yar Combat Form]]
**[[Flood Combat Form#Unggoy and Kig-yar Combat Forms|Unggoy Combat Form]]
**[[Active Camouflage Combat Form]]
*[[Flood Carrier Form|Carrier Form]]
**[[Shielded Flood Carriers|Shielded Carrier Form]] {{C|cut}}
**[[Flood Combat Carrier Form|Combat Carrier Form]] {{C|Simulation only}}
*[[Flood Infection Form|Infection Form]]
|affiliation=''Inferi'' genus   
|notable=*[[Gravemind]]
*Notable Infected Individuals
**[[Jacob Keyes]]
**[[Prophet of Regret]]
**[[Prophet of Truth]]
**[[Wallace Jenkins]]
|othernames=*"Flood" ([[Human]] and [[Covenant Empire|Covenant]])   
*"''Inferi redivivus''" ([[Forerunner]])
*"Parasite" ([[Covenant Empire|Covenant]] and [[Forerunner]])
*"Shaping Sickness"
*"The Plague" 
}}
{{Quote|One single Flood spore can destroy a species.|[[Rtas 'Vadum]]|[[Halo 3]]}}
'''The Flood''' (Latin '''''Inferi redivivus''''',<ref name="bestiarum" /> meaning '''"the dead reincarnated"''')<ref>[http://carnage.bungie.org/haloforum/halo.forum.pl?read=865888 '''Halo.Bungie.Org''': Updated and abbreviated list, errors amended]</ref> '''the Parasite,'''<ref>'''[[Halo 2]]''', level ''[[The Heretic]]'': Thel 'Vadamee: ''"Noble Hierarchs, surely you understand that once the parasite attacked-"''</ref> as they are known to the [[Covenant Empire|Covenant]], and '''''the Shaping Sickness''''' by the Ancient Humans<ref>'<nowiki/>''[[Halo: Primordium]]''' - '''''<nowiki/>'''p.53'''</ref>, is a conglomerate of highly virulent parasitic organisms that can reproduce and grow by consuming sentient life forms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient life in the galaxy, notably the [[Forerunner]]s, during the 300-year-long [[Forerunner-Flood war]].<ref>'''[[Halo: Combat Evolved]]''', level ''[[Two Betrayals]]'': First cut scene, referenced by [[343 Guilty Spark]]</ref> The Flood presents the most variable faction in the trilogy, as it can infect and mutate [[Human]]s and Covenant species, such as [[Sangheili]] and [[Jiralhanae]], into [[Flood Combat Form|Combat Forms]]. They are widely considered to be the greatest threat to the existence of life, or more accurately biodiversity, in the Milky Way galaxy.

==Origins==
As the [[Forerunner-Precursor war]] drew to a close, the few remaining [[Precursor]]s that were pushed out beyond the edges of the galaxy to [[Path Kethona]] sought vengeance against the Forerunners. A few Precursors were spared by the Forerunners, while others rendered themselves down into a molecular dust which could regenerate into their past forms under proper stimuli. Over time, this dust became so corrupted by age that it could only cause horrific disease and mutation in those exposed to it. The Precursors, whose forms had been transmuted into the dust, were driven utterly insane by the rebellion of their creations, and thus embraced their growing corruption, and became part of it, altering themselves into the Flood so that they might destroy the Forerunners and their empire as a final act of vengeance against them.<ref>''[[Halo: Silentium]]''</ref> They sent several ships containing this powder to Forerunner controlled planets.

==Background==
The Flood itself is little more than an extremely adaptable, dangerous, and highly infectious parasitic life form that can differentiate into a multitude of varying “species,” or forms.

They were first encountered in the [[Milky Way galaxy]] by the Ancient Human empire some 10,000 years prior to the Forerunner-Flood war. The earliest known instance of the highly infectious viral supercell later known as The Flood Spore to be encountered came in the form of a fine powder, contained in cylinders that had been found aboard several ancient, crashed starships. This powder was actually the desiccated remains of Precursors who had escaped the Forerunner purge. 

Early experiments conducted on this powder showed that it had the potential to produce favorable psychotropic effects in some lower animals, but not in humans or [[San'Shyuum]]. For all intents and purposes, it was completely harmless. 

The primary animals affected by the powder were popular pets in human society at that time: the [[Pheru]], a species of lively and gentle creatures native to the planet Faun Hakkor.

Very small quantities of the powder induced changes in the Pheru that improved their domestic behavior and made females more fertile. Soon, the San'Shyuum came to adopt the custom of keeping the Pheru as pets. For centuries, humans and San'Shyuum bred and powdered these animals without anny obvious ill consequences. However, the powder soon attached itself to key points in the genes of the Pheru and began to change them, which manifested itself as a peculiar growth found on a third of the powdered Pheru.

A kind of loose, soft fur grew between the shoulders of the pets. Other Pheru were soon found grazing on these companions, consuming their fur, and on occasion even consuming the animals themselves. This was odd, as Pheru were naturally herbivores. This seemed to activate some sort of signal for expansion. Within a very short time, the Pheru were producing far less attractive growths. Flexible striped rods sprouted from their heads, which in turn were also consumed by fellow Pheru, causing abortions and unnatural births.

But this was only the surface of the growing infestation.<ref name=":0">'''[[Halo: Cryptum]]''', Page 270</ref> The Flood spread from the Pheru to Humans, specifically through the humans who ate the Pheru as food.  These new vectors forced their fellows to consume infected tissue and spread the infection to whatever they touched. In time, whatever they discarded—limbs, tissue—could also spread infection.<ref name=":0" /> The Flood continued to mutate and evolve and fought a war with both the [[Prehistoric human civilization|Ancient Human Empire]] and the San'Shyuum.

Unknown to the Forerunners, the Human-San'Shyuum alliance had developed a way to combat and eventually defeat the Flood. By manipulating key strands of the Flood's genetic buildup, and infecting a third of the human population with this code, then placing them directly along the pathway of the Flood, the humans were able to alter the resultant Flood, causing it to turn upon itself. As a result, the Flood began to tear itself apart on a galactic level, further spreading the Human-San'Shyuum genetic weapon, and causing the Flood to wipe itself out.

Knowing that it faced its own extinction, what remained of the uninfected, stable specimens fled the galaxy aboard a commandeered vessel and remained dormant for thousands of years, biding their time.

The Forerunners learned of the peril and as a precaution designed and subsequently constructed the Halo Network should the threat one day return. As expected, 10,000 years later, the Flood returned and began its conquest of total galactic domination.

After exhausting every other strategic option, the Halo Array was fired, resulting in the elimination of the original source of the Flood infestation and the death of every sentient being in the Galaxy, effectively removing both the source and its entire food supply.

The Flood exists in a fairly stable state in the absence of a [[Gravemind]]. They lack the ability to coordinate their efforts, but the Flood will spawn spontaneously and spread by infecting sentient life forms, specifically those that are self-aware and capable of introspection. This stage of a Flood outbreak is known as the [[Feral Stage]].

During its Feral Stage, the Flood is only capable of local coordination through use of pheromone-based communication. Research indicates that the content of said messages are of very limited complexity. Individual [[Flood Combat Form|Combat]] and [[Flood Carrier Form|Carrier Forms]] have access to the skills and memories of their host; however, once an outbreak establishes a viable Gravemind, the [[Coordinated Stage]] begins, and it is at this point the Flood becomes truly dangerous.

With the formation of a Gravemind, the assimilated intelligence and memories of consumed hosts become part of a collective consciousness, which is capable of strategic thinking and commanding individual Flood forms in an organized manner.

A Gravemind (like the Proto-Gravemind that gave rise to it) is the fusion of pure Flood Super Cells and the complex neuro-system of hundreds or thousands of sentients. The control it exerts over the more mobile forms it creates is likened to an entity's control over its own limbs. Therefore, during the Feral stage, control is localized at the individual level, but once an outbreak enters the Coordinated stage control is assumed by, and broadcast from, the central compound intelligence.

Although little data exists from any adequately controlled experiment, data gathered in the field suggests that if a mobile form is removed from the influence of the Gravemind coordinating its group of origin, it will revert to a Feral Stage. Insufficient data exists on whether mobile forms of one Gravemind would be influenced by another Gravemind, though logic dictates this would be the case. However, because individual Graveminds are more or less identical and are all striving for the same goal, it is likely that a distinction would be impossible to detect.

The [[Bestiarum]] states that the society of the Flood closely resembles a "utopian socialist ideal as there is no wealth, no poverty, no want, no crime, and no disease; every individual works for the advancement of the Flood as a whole." It also states that it "is unfortunate that their one overriding goal is to [butcher and consume] every other life form in the galaxy."<ref name="bestiarum" />

==History==
===Human-Flood war===
{{main|Human-Flood war}}
[[File:Human ship firing (2).jpg|thumb|A ship of [[ancient humanity]] fires upon a seemingly infected other ship.]][[File:Human ships and fighter.jpg|thumb|left|Several planets were lost to the Forerunners simply from being cleansed of small Flood infections, being attacked by the [[Ancestors]].]]The Flood first entered the galaxy from the Large Magellanic Cloud at the edge of the Milky Way roughly 110,000 years ago, contained in cylinders in unmanned vessels where they were kept as a powder. A considerable amount of time passed between their initial discovery by prehistoric humans and their evolution into their current form. The Flood were eventually able to infect Human worlds, sparking the [[Human-Flood war]]. The Humans, who were losing worlds to the Flood, continually searched for new territory and went as far as to take Forerunner occupied planets or completely glass them if Flood infestation was detected. The Forerunners, seeing this as an act of aggressive imperialism rather than an attempt to hold to the Mantle, retaliated, precipitating the [[Human-Forerunner war]]. However, the Humans successfully developed a 'cure' for the Flood infection and were able to eradicate it (but not before destroying their research) before falling to the unmolested and more powerful Forerunner military. It would be thousands of years before they were again able to threaten the galaxy.

Unbeknownst to the humans, they had not actually found a cure; the Flood had chosen not to infect the humans instead; the former Precursors allowed the idea of a cure to gain credence, so that in the future more efforts would be diverted to attaining it rather than simply excising the infection. When the Flood returned, the Forerunners, who devolved and destroyed ancient humanity, were entirely unprepared to challenge the Flood properly.

===Forerunner-Flood war===
{{main|Forerunner-Flood war}}
[[File:Ships flood.png|thumb|Flood infested Forerunner ships to spread their conquest to more worlds.]]
{{Quote|It fed on intelligent life and in doing so, became ever more intelligent itself. The Flood was unique, it used [[forerunner|their]] very strength against them.|Cortana on the Forerunner-Flood war.}}

{{Quote|For everyone there is a time like this, and for you that time is now. Know this, relentless enemy, killer of our children, Lord of Admirals... Soon we will face the enemy you have faced... And we are afraid.|[[Shadow-of-Sundered-Star|Ur-Didact]] to [[Forthencho]] on his death bed.}}

More than 100,000 years prior to [[2552]],<ref>'''Halo 2''', level ''[[Sacred Icon]]''</ref> the Flood encountered a Forerunner research expedition on the planet [[Seaward|G 617 g1]]. Initially, the Forerunners severely underestimated the potential threat this new life form posed, using tactics more suited to disease outbreak than true warfare. By the time the infection had begun to spread, the Flood had continued to grow in biomass and were a deadlier and more intelligent foe. The Flood used unarmed civilian assets to penetrate planetary defense groups, first striking at the Forerunner-held planet of [[LP 656-38 e]], completely bypassing the Forerunner Orbital [[Fleet]] and infesting the planet below. They soon moved to similarly infest the planet [[DM-3-1123 b|DM-3-1123b]]. The sheer numbers of Flood forms on the planets overcame Forerunner ground forces. While at first the Flood demonstrated a lack of even basic group tactics, they had numerical superiority; billions of forms dedicated to the assault, and every member of the Forerunner population a potential Flood host. In desperation, the Forerunner's armada was ordered to immediately commence full planetary bombardment on infested worlds, although at a great cost - many Forerunners were unable to be evacuated before bombardment commenced, leaving the Forerunner military with shallow victories. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers appeared to activate localized weapons of mass destruction, effectively committing mass suicide, but stopping the growing infestation.

[[File:H4-Terminal-Flood-Fleet.jpg|left|thumb|The [[Mantle's Approach]] prepares to deploy [[Promethean Knight]]s to destroy Flood-infected [[Ancestors|ancient human]] ships.]]Thereafter, the Forerunners realized that ordinary naval tactics were unable to stem the growing tide of the Flood, and decided to pin their hopes of defeating the Flood on super-weaponry. While the Forerunners initially deployed the [[Z-1500 Automated Systems Drone|Sentinel]]s, robotic drones to fight against and contain the Flood, almost immediately afterward, the [[Forerunner Fleet Command]] contemplated "Premature stellar collapse"; using naval battle groups to destroy planetary star systems' primary stars in supernova that would engulf infested worlds and prevent any possibility of Flood infection.

The Flood grew exponentially, spreading from system to system with the intent to infect the Forerunner population rather than wage war with Forerunner battle groups. The Flood had reached sufficient proportions to create 'Key Minds' Enormous instances of Gravemind with a vast computational capacity that challenged even the Forerunners most powerful machines. The tide began to turn against the Forerunners; the overwhelming computational power of the Gravemind allowed it to outmaneuver the Forerunner Fleets. The Gravemind convinced the [[Contender-class Artificial Intelligence]] [[032 Mendicant Bias]] to join its cause, along with any other AI it could directly interface with. The Forerunners suspected that the Flood had a logic altering informational virus that was effective against all of their AI. Mendicant Bias became rampant, and defected to the Flood cause to destroy its former masters. The Flood began manipulating Precursor technology—specifically the Star Roads: enormous devices that warped spacetime, both destroying Forerunner ships caught in its systems and preventing them from jumping to Slipspace and escape.

The Forerunners began their "Conservation Measure" in force, researching the Flood while constructing an [[The Artifact|Artifact]] on [[Earth]] and other worlds to generate a [[Slipstream Space|Slipspace]] portal leading to [[Installation 00|The Ark]], which they used to efficiently save as many sentient beings as they can while cataloging and preserving parts of the planet's ecosystems. 

The Forerunners employed many, many measures, including intricate naval tactics in an attempt to contain the Flood, which ultimately failed. Finally, the Forerunners were forced to a Pyrrhic solution, utilizing twelve ring-like super-weapons, known as the [[Halo Array]], across the galaxy, later all but one being destroyed, and [[List of seven references in Halo|Seven]] survived to their activation) that had been previously built under the machinations of the Master Builder, Faber. Because the Flood were parasitic, and their survival was directly linked to the presence of potential hosts, the Forerunners reasoned that eliminating all potential Flood hosts, that is, all sentient life forms in the galaxy, would render the parasite harmless by way of it having nothing to infect, as explained by the Forerunner AI 343 Guilty Spark. After all countermeasures failed, the Forerunners activated the Halo Array in 97,445 B.C, unleashing galactic destruction, and destroying almost all sentient life forms in the Galaxy, sometimes including the Flood itself. However, the human race and many other species throughout the galaxy had been indexed before the purge by [[First-Light-Weaves-Living-Song|The Librarian]]. Population samples were preserved on the Ark, or within the [[Shield World]]s, and out of range of the Halo Array.

The Flood was killed or contained, unable to reproduce and grow due to lack of hosts, and eventually died out. The only surviving specimens were contained in state of the art, high-security Forerunner research facilities, such as the Halo Installations and the [[Threshold]] [[Threshold gas mine|Gas Mine]]. After the Flood threat was destroyed, [[Keyship]]s were dispatched, and reintroduced the indexed species to their origins, sparking life back into the Galaxy again.

===Battle of the Apex===
{{main|Battle of Shield 0459}}

Thousands of years after the end of the Forerunner-Flood war, the human ship {{UNSCShip|Spirit of Fire}} jumped from Arcadia to follow the signal of [[Ellen Anders|Professor Anders]], who had been captured by the Covenant. They were led to an unknown planet, which happened to be a Forerunner Shield World. Containment measures on this planet had failed catastrophically, and the entire surface of the planet was choked with Flood biomass. Ground forces were deployed to investigate the presence of unknown hostiles sighted by the ship's radar. After fighting their way through Covenant forces, [[John Forge|Sergeant Forge]] and several Marine squads were attacked by the Flood.

The surviving UNSC forces, which included Sergeant Forge, were ordered to recover some [[M312 Heavy Recovery Vehicle|Elephants]] that were surrounded by Flood forms. After one was rescued, the small squad made their way into a site that would become a base. They then assembled a larger force to rescue the other soldiers. Following their successful rescue, the UNSC forces prepared to engage the Flood.

[[SPARTAN-II Program|Spartan-II]] [[Red Team]], while searching for [[Ellen Anders|Anders]], was ambushed by several Flood forms, which forced their Warthog to crash. The remaining forces at the nearby base, led by Sergeant Forge, moved out to rescue the ill-fated team. As they attempted to rescue the Spartans, they soon discovered that the Flood was creating a Proto-Gravemind, and that it may have been jamming the tracking signal that Anders was giving out. On their way to the soon-to-be Gravemind, they encountered several Flood Colony forms, and found that by destroying them, the Proto-Gravemind would become weakened. Once they destroyed the Proto-Gravemind, [[Serina]] found a signal, which was believed to be Anders'.

Captain Cutter then ordered all UNSC personnel on the planet to return to the ''Spirit of Fire''. These orders were impeded by the Flood, which attempted to prevent the humans' retreat, but the Flood were eventually held back long enough for the UNSC forces to return to the ''Spirit''. The Flood attacked the ''Spirit'' as it entered the interior of the Shield World, but was wiped off by the ship's crew, Sentinels, and the Shield World's cleansing rings. All the Flood forms on the Shield World were destroyed when Sergeant Forge used the ''Spirit of Fire''{{'}}s [[Shaw-Fujikawa Translight Engine]] to destroy the Shield World's internal sun, obliterating all Covenant and Flood forces on the Shield World. ''Spirit of Fire'' narrowly escaped by performing a hastily-calculated slingshot maneuver around the exploding sun in order to shoot off into space. Without the ''Spirit's'' Slipspace Engine, however, the ship was lost in space, with humanity's only knowledge of the Flood lost with it. However, a number of Flood forms survived the destruction of [[Shield 0459]] by infiltrating ''Spirit of Fire'' and had seemingly taken over the ship by 2558.<ref>''[[Halo: Escalation]]'' – Issue #6</ref>

===Battle of Installation 04===
{{main|Battle of Installation 04}}
====Release====
{{Quote|The only way to stop the Flood is to starve them to death.|[[Cortana]] to [[John-117]] and 343 Guilty Spark.}}

[[File:Flood attack.jpg|thumb|[[John-117]] fends off the oncoming Flood Combat Forms and [[Flood Infection Form|Infection Forms]] on Installation 04.]]

Although the first firing of the Halo Array starved large numbers of the Flood, samples of parasitic specimens were kept alive (perhaps in stasis) on various Forerunner outposts for research and further scientific study. One such research site was located on [[Installation 04]]. This proved to be a grave, irreversible mistake, as both Humanity and the Covenant stumbled upon the abandoned ring thousands of years after the firing of the Array and the destruction of the Forerunner and most of the Flood. Cortana, aboard UNSC ''Pillar of Autumn'', made what was presumed to be a "blind" jump through Slipspace (although it was later revealed that she used translated coordinates from a Forerunner artifact found on [[Sigma Octanus IV]]) to evade Covenant pursuers. This led both species to Installation 04, and in turn, the Flood that were kept there. After the crash of the ''Pillar of Autumn'' on the Halo Ring, the surviving crewmembers dug in for what they believed would be a long and brutal guerrilla campaign against the Covenant forces on Halo.

Meanwhile, the pursuing Covenant cruisers (among them ''[[Truth and Reconciliation]]'') amassed their ground forces on the surface, and soon discovered an underground Forerunner [[Flood containment facility|Flood Containment Facility]] in the swamps of Halo. Due to the secure nature of the facility, the Covenant came to use it as a fortified base of operations and an area for storing weaponry, oblivious to its true nature. Though some of the Flood managed to escape, the terrified Covenant soldiers eventually pushed the Flood back in.

After [[Captain (Navy)|Captain]] [[Jacob Keyes]] was captured by the Covenant for interrogation, he caught wind of this weapons cache and, upon his rescue, organized [[Fire Team Charlie]] to seize it. When the team arrived at the swamp however, they found the facility was completely devoid of Covenant personnel. However, upon takeoff, a Pelican Dropship, (Call Sign [[Victor 933]]) which had transported them to the base, crashed into the swamp as a result of enemy action. Then, immediately after the crash, the pilots were able to record a message about their attackers, calling them "Unknown Hostiles." These "Unknown Hostiles" were actually the Flood, and shortly afterward, they were seen by [[John-117]] creeping throughout the swamp. At first, when he saw the Covenant retreating out of the Forerunner structure running from [[MA5B Individual Combat Weapon System|Assault Rifle]] fire, he believed them to be allied forces. However, after he opened the door that Captain Keyes had unlocked, he learned the truth through Private Jenkins' recording: this was no Covenant weapons cache. Numerous Infection Forms attacked John-117 after reading the recording, but he managed to fight his way out of the facility, leading the remaining Marines of Keyes' squad to a drop zone and encountered [[343 Guilty Spark]]. Guilty Spark then promptly teleported him into the Library and informed him, "The Flood is spreading. We must hurry!"

The few surviving Covenant soldiers left were scattered and disorganized, and put up little resistance to the new Combat Forms created from numerous Covenant corpses. It is presumed that the Covenant occupants of the research facility had moved through the structure in hopes of finding Forerunner artifacts, and had inadvertently released living Flood specimens. After witnessing the capabilities of the Flood, the Covenant forces locked the facility down and evacuated all personnel who survived the Flood's escape. Keyes and his team were unaware of this fact, however, and continued to delve deeper into the facility, ultimately coming across one of the unopened Flood Containment Chambers. Believing it was a weapons cache of some sort, they unlocked the door, and were soon attacked and assimilated by the Flood inside. With a large supply of "edible" sentient life now occupying the ring, the Flood began to multiply and infest most of Halo. Soon, the infestation proved too great even for Halo's automated military force, the Sentinels, to handle. The ring's commanding AI, 343 Guilty Spark, was forced to call upon a "''[[Reclaimer]]''" (First Staff Sergeant Marvin Mobuto, and subsequently John-117 when Mobuto failed) to activate the array once more. John-117 successfully retrieved the [[Index]], the item required to activate the ring and returned to Halo's Control Center.

These events brought the knowledge of the Flood to the UNSC. In a war of losses, this was another grim revelation for humanity.

====Assault====
[[File:Infectionforms Halo1.jpg|thumb|[[Flood Infection Form|Infection Forms]] feeding on dead corpses. Shown in one of the Chambers leading to a Pulse Generator in the Level [[Two Betrayals]], in ''Halo: Combat Evolved''.]]

After the Flood's escape, the Covenant forces, who were widely stretched across Installation 04, were heavily engaged, and were being overwhelmed by Flood forces. While in some engagements, the Covenant were able to hold the line by using vehicular and air support, most of the weaker Covenant soldiers such as [[Unggoy]] succumbed easily to the Flood, and the few Sangheili who remained were outnumbered, killed and reanimated. The [[United Nations Space Command|UNSC]], having a much smaller presence on the Ring, found out about the Flood's might a short while later, when a UNSC [[Orbital Drop Shock Trooper|ODST]] force engaged with the Covenant near [[Alpha Base (Installation 04)|Alpha Base]], was ambushed by the Flood and suffered heavy casualties. However, one of the attacking combat forms, [[Wallace Jenkins]], who was one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by a weak and elderly Infection Form whose powers of infection had been dulled by years of hibernation, making Jenkins' transformation into a Combat Form incomplete. Jenkins was still able to resist the pervasive Flood entity in his mind, and although incapable of articulating speech properly and with a physiologically devastated body, Jenkins was still able to betray the Flood through hand gestures during an interrogation with [[Major (UNSC)|Major]] [[Antonio Silva]], informing the UNSC Marine battalion that the Flood planned to assault the UNSC base from a series of underground catacombs underneath their base. In response to this intelligence, the UNSC moved to secure them, and prevented the Flood from staging another attack.

By the time of these developments, Flood dominance over the Covenant was assured. The Sentinels were unable to contain the Flood, and the Covenant and UNSC were sustaining heavy losses and rapidly losing ground. Meanwhile, the Flood were occupying whole areas formerly held by the Covenant, although Covenant and Flood forces continued to wage war on the massive ice plains of the Ring. Although the Covenant had the benefit of [[Type-26 Anti-Infantry Stationary Gun|Shade]] class stationary [[plasma]] turrets and vehicular/aerial support, the Flood had captured both UNSC and Covenant weapons and personnel, most notably [[M19 SSM Rocket Launcher|M19 Rocket Launchers]] and massive numbers on their side.

The Flood, however, were not unintelligent. One of the UNSC personnel captured in the Marine force to the ex-Covenant arms cache was Captain [[Jacob Keyes]], a brilliant naval strategist of the [[UNSC Navy]]. When he was infected and initially used as a Combat Form, the Flood realized Keyes was holding vital information and relegated him as biomass for a Proto-Gravemind. Keyes knew the location of [[Earth]], the birthplace and major planet of the human race, a planet with tremendous possible assimilation opportunities for the Flood. He was merged with at least four other victims into a massive, engorged Proto-Gravemind aboard the {{Class|CCS|battlecruiser}}, ''Truth and Reconciliation'', a Covenant warship that the Flood were relentlessly attacking. While he was in the brain form state, the Flood began to comb his memories. Keyes successfully withheld information about Earth by constantly accessing information available on his [[Command Neural Interface]] that was not vital, such as his name, rank and serial number. While proceeding through several hangars, Cortana saw the Flood amass both Unggoy and Sangheili corpses for biomass. Learning of the truth of the rings by Cortana, John refused to help 343 Guilty Spark and Cortana suggested an alternative to stop the Monitor from firing the rings by destroying the ''Pillar of Autumn''. However, before they could do so, they had to get Keyes' Command Neural Interface.

The Covenant Flagship ''Truth and Reconciliation'' was a major point of contention between the [[Covenant Special Operations]] units and the Flood. The Flood had likely entered the warship previously, decimating most of the Covenant crew and garrison on board, leaving only isolated pockets of resistance, mostly small numbers of terrified Unggoy and a few Sangheili aboard the vessel. However, below ''Truth and Reconciliation'', in a series of rocky canyons, the Covenant were much stronger in force, with large numbers of Sangheili supported by Unggoy, Kig-Yar, and [[Mgalekgolo]], fighting off the Flood. The Covenant High Command on Installation 04, however, were terrified by the Flood presence. Because the {{Class|CCS|battlecruiser}} had been damaged in a previous space battle with {{UNSCShip|Pillar of Autumn}}, it had been grounded, awaiting repairs for at least two days. After the Flood were discovered to be aboard the battle cruiser, the Covenant High Command sent a Special Operations strike force to ''Truth and Reconciliation'' to commence immediate emergency repairs, prepare it for lift-off into space and neutralize the Flood. 

Arriving at the ''Truth and Reconciliation'', John and Cortana discovered that Keyes had been assimilated into the Flood, so John lunged into Keyes' skull to rip the interface out, denying the Flood Earth's location.

Sometime after John left the cruiser, the remaining UNSC Marine forces on the ring staged an assault on the Flood-held ''Truth and Reconciliation'', and managed to neutralize most Flood Forms on the cruiser in a swift assault, and the captain, [[Major]] [[Antonio Silva]], violated the [[Cole Protocol]] by having the ship leave immediately after emergency repairs in an attempt to gain glory by bringing a Covenant cruiser to Earth rather than sweep the ship to ensure no Flood forms were hiding inside. However, there was tremendous risk if the improperly checked vessel traveled to Earth, if there was even one form aboard, "''Earth could fall,''" according to UNSC [[Artificial intelligence|AI]] [[Wellsley]], because of the Flood's exponential parasitic potential. Against orders, Marine officer [[Melissa McKay|McKay]] destroyed the warship with whatever Flood forms were still aboard, killing hundreds of UNSC personnel, including an infected Jenkins, but saving Earth and possibly billions of lives from the Flood.

====Defeat====
The '''Flood''' appeared to be interested in seizing space-capable vessels, in the hope of spreading itself away from the Installations and into the stars: they attacked UNSC ''Pillar of Autumn'' and CCS ''Truth and Reconciliation'' for this reason. After the UNSC cruiser's crash-landing onto the ring, and after engaging over fifteen enemy warships, it had been taken by the Covenant, although a UNSC raid had managed to temporarily retake the hull. On the third and final day of the Battle of Installation 04, however, the Covenant were in control of the crashed warship when a Covenant patrol was ambushed by the Flood and infected; when it returned into the vessel, the Covenant garrison was attacked by Sangheili Combat forms. The combat forms were able to scatter before they could all be destroyed, and a steady trickle of Flood continued to enter the Pillar of Autumn through vents and openings in the hull, waging a tedious guerrilla battle against the Covenant security force on board. Eventually, the [[Z-1500 Automated Systems Drone|Sentinel]]s came on board, overwhelming the dwindling Covenant forces as the Flood began to attack the warship in full force, in the end almost completely obliterating the Covenant presence, and severely limiting the Sentinels on board. Despite their best efforts and enormous numbers, the Flood, Covenant, and Sentinels were unable to stop [[John-117]] from detonating the wrecked warship's reactors, and escaping in a [[GA-TL1 Longsword-class Interceptor|C709 Longsword-class Interceptor]]. The resulting thermonuclear detonation severed the ring world, destroying it and its life support capability to annihilate all sentient life, and simultaneously killed all remaining Flood on Installation 04, along with any other life forms that were on the ring.

===The Mona Lisa Incident===
{{Quote|Do they eat their own, cause that's what it looks like.|Burgundy}}

Sometime after the destruction of Installation 04, the UNSC ''[https://halo.fandom.com/wiki/UNSC_Red_Horse Red Horse]  ''was sent to investigate the remains of the ring. Despite being sent by the [https://halo.fandom.com/wiki/Office_of_Naval_Intelligence Office of Naval Intelligence], the ship's orders were vague, with the only instructions being to investigate the debris. During the search of the debris field, the ship intercepted and secured an escape pod. Sergeant Lopez and her marine unit (a marine detachment on the ship) were then ordered to inspect the pod. They managed to find a survivor who eventually died from blood loss, but not before revealing in a delirious manner that he was an escaped prisoner from the ''Mona Lisa'', a civilian prison ship.

The ''Red Horse''{{'}}s commander, [https://halo.fandom.com/wiki/Tobias_Foucault Tobias Foucault], summoned Sergeant Lopez to the meeting room, along with Rebecca, the ship's AI. They explained to Sergeant Lopez that it was extremely strange that a civilian ship like the ''Mona Lisa'' could be in such a classified location, and even more strange that it would be roaming space without an escort, since typical prison ships rely on an escort in case of a prison riot. After the mission briefing, Lopez and her team were then dispatched via Pelican to investigate the ''Mona Lisa. ''As they approached the ship, Lopez introduced each of the marines on board, since some were new members. When the group arrived at the ship, Rebecca informed them that a Covenant capital ship had been spotted. Although its was still distant and unware of their presence, they were advised to tread carefully and not draw any attention to themselves.

Once the group boarded the ''Mona Lisa'', they encountered a group of unarmed naked Elites in the poorly lit cargo bay. Despite their numbers, the marines easily killed them all but were confused by the strange lack of weapons and equipment on the Elites. The Marines assessed the area, finding no prisoneers, crew members or any signs of human life. Lopez suddenly noticed that Rabbit, a member of their squad, had disappeared. The sergeant ordered another marine, Cranker, along with their pilot, Burgundy, to stay behind while they searched for Rabbit. The group proceeded down a hallway splattered with blood, although there were no plasma marks or bodies. 

They entered a storage room, only to find Rabbit, dead, being stomped to a bloody pulp by another Elite. Lopez and the marines opened fire on it, shooting the elite to a similarly bloody pulp. As they secure the room, Benti, the medic sees two figures run past the hallway towards the cargo bay. Lopez warned Cranker and Burgundy, before she noticed a strange growth on Rabbit's body. Suddenly, they heard a screech, and Lopez radioed Cranker to check up on him. He tells her that the elites never showed, but a growl can be heard from his radio. Suddenly he is overwhelmed by something as he screams, and then his radio goes silent.

Burgundy searched the cargo bay for Cranker from within the ''Pelican'' via its rear cameras, but only saw his assault rifle's flashlight in the dark. She then noticed one of the Elite corpses being dragged off, and was contacted by Rebecca, who she patched through to Lopez. Lopez requested reinforcements, but Foucault denied the request, instead ordering Lopez to initiate the Cole Protocol and destroy the ''Mona Lisa'''s nav data, before their communication broke up.

After relaying the commander's orders, Lopez ordered Burgundy to remain in the ''Pelican ''and keep an eye out for Cranker. She then split the group into two squads, sending five marines lead by Benti to the engine room to clear the backup nav system while Lopez headed to the bridge with a few other marines to clear the primary nav systems.
Lopez's group made a brief stop in a med bay to treat a wound one of the Marines had sustained earlier. The squad noticed a locked safe room, and after some persuasion, convinced the room's occupant, Dr. John Smith, to come out. Lopez asked him what happened to the ship and doubts his explanation that the prisoners escaped and overwhelmed the guards. She then asked him why there were Covenant prisoners on the ship, but Smith claimed to have no idea why. Lopez finally asked what the ship was doing there, but Smith had no knowledge of the reason. After she finished questioning him, Lopez informed Smith that the group was headed to the bridge to delete the ship's nav data. Smith said they needed to leave the ship immediately, but Lopez forced him to join the group as they continued their mission.

Meanwhile, Burgundy remained in the hangar waiting for Cranker. At some point she sees Cranker emerge from the hallway. She proceeded to open the ''Pelican'''s door to greet him before noticing that he is covered with blood and groaning strangely. Before she could react, he knocked her to the ground with a claw and dragged her away.

Benti's team found a maintenance shaft that lead to the recycling plant. The group proceeded through the shaft, hearing groaning and growling in the distance. They then spotted a group of figures chasing them, so they fled until they reached a junction. The group turned around to face their pursuers, a swarm of fleshy, twisted monsters - Flood.

Elsewhere, Lopez and her team had reached a cafeteria and found a mutilated corpse. A marine asked Lopez what it was, and she responded by asking Smith if he knew what it was. He said that it was a virus from one of the Elites. He also stated that it pushed the infected into crazed aggression.

Back in the maintenance shaft, Benti and her team opened fire on the Flood forms, but quickly noticed their fire was largely ineffective. Benti ordered the group to fall back into a room while she threw a grenade into the shaft, which crushed the Flood forms but also killed a Marine, as well as partially flooded the room. A Combat Form rose from the water and slashed another Marine before Benti killed it. While examining the Marine's wound she noticed and cut out a small maggot-like thing. The group carried the wounded Marine onward, but Gersten, another marine, notices the wounded soldier worsening.

As Benti's group continued onward, the wounded Marine suddenly attacked the group, biting Gersten before being killed by another Marine, Clarence. The group hurried onward and reached the next room, where they found another naked Elite wielding a cricket bat.

===Threshold Gas Mine===
{{Quote|Stand firm! The Flood is upon us! |[[Rtas 'Vadum]]ee}}
The Flood were also kept in the [[Forerunner]]-constructed Threshold Gas Mine, in one of the three peripheral arms of the station. A small [[Covenant Special Operations|Covenant special forces strike team]] of three [[Phantoms]] loaded with [[Special Operations Sangheili]] and [[Unggoy]], led by [[Special Operations Commander of the Covenant|Special Operations Commander]] Rtas 'Vadumee and [[Thel 'Vadam]]ee, were dispatched by the [[Hierarchs|High Prophets]] to this station. This was to "silence" [[Sesa 'Refumee]], the leader of the Heretics stationed within the mine encouraging dissent against the Prophets, who had learned the truth of the rings from 343 Guilty Spark, who had survived the explosion of his own installation. As the Heretics were unable to stop the Covenant, Sesa unleashed the Flood in the hopes that they could stop them.

[[File:Dormant flood.jpg|thumb|Dormant Flood in receptacles housed in the Gas Mine.]]

During Thel 'Vadamee's pursuit of Sesa 'Refumee, the strike team entered a laboratory,<ref name="H2Guide">'''[[Halo 2: The Official Strategy Guide|The Official Halo 2 Strategy Guide]]''', pages 92-95</ref> in which they were locked into while the Flood was released in an attempt to quell the attackers. The plan failed, and instead allowed the Flood to infiltrate other parts of the station, killing both Covenant and Heretics alike. The remaining Covenant strike team then continued through a large elevator that ran in the spine of the Flood-infested arm. While presumably used to transport gas conduits, its walls were rampant with Flood, and it ferried canisters full of Infection Forms. The tenuous, agonizing elevator was nicknamed "A Descent Into Hell."<ref name="H2Guide" /> The Flood began making its way up the station, overtaking the Heretic forces and forcing 'Refumee to seal himself inside safe section of the station. This action forced Thel 'Vadamee to sever the cables connecting the station to the higher atmosphere. 343 Guilty Spark later explains this was a strategic placement by the Forerunners, as afterwards the facility would fall into the [[Threshold|planet's]] solid Nitrogen core.

===Battle of Installation 05===
[[File:H2 Index.jpg|thumb|The Flood were in search of the [[Index]], a device which would deprive [[United Nations Space Command|UNSC]] and Covenant forces from the privilege of assured destruction of the parasite.]]
{{main|Battle of Installation 05}}
Unlike on Installation 04, where the Sentinels had contained the Flood until the Covenant began to meddle with the Forerunner containment measures, on Installation 05, the Flood had been in continual war with the Sentinels since the firing of the Halos 100,000 years ago, with a [[Gravemind]] leading the forces. It is unknown how. Like on Installation 04, the fresh suitable host bodies of a newly arrived Covenant armada and a UNSC strike force fueled the Flood's exponential parasitic capabilities, and Flood infestation increased at a surprisingly rapid rate, and within hours, Sentinel forces were being overrun, causing a risk that the Ring would fall to the parasite.

In order for the activation of the Halos and the facilitation of the ultimate Flood containment measure to occur, a [[Reclaimer]] had to retrieve [[activation Index|the Index]] from [[Installation 05]]'s own [[Library]], making [[the Library]] a high-priority target for the Flood: if it could control the Library and prevent any Reclaimer from entering, it would make itself essentially invulnerable. Installation 05's Library was nestled in a tenuous series of wintry and snowy canyons and boasted three levels of defense: an exterior [[Sentinel Wall]] kilometers in the distance, an underground honeycombed facility that produced large numbers of Sentinels (that impeded any invaders from advancing), a [[Containment Shield]] (an [[energy shielding|energy shield]] that denied access by any means to the Library until it was deactivated), and a [[Quarantine Zone]] (a type of biological "airlock" to prevent the Flood from spreading any further). This multi-tiered defense was patrolled by hundreds of Sentinels and [[Sentinel Major]]s, and several [[Enforcer]]s.

However, two external forces, the Covenant and the UNSC, wanted the Index as well: the Covenant, to activate the Halos and set off the misinterpreted "sublimation event" of the [[Covenant religion|Great Journey]], and the UNSC to secure the Index and prevent the Covenant from doing so. The Covenant sent a Strike Team of Special Operations [[Sangheili]], commanded by [[Rtas 'Vadum]]ee, and led into battle by Thel 'Vadamee, to retrieve the Index in a multi-staged assault. However, the Covenant first needed to deactivate the Containment Shield by disconnecting its power generator in the center of the Sentinel Wall.

Previously, a force of [[Jiralhanae]]s, [[Unggoy]]s, and [[Kig-Yar]]s had been dispatched to complete that operation, but they were overwhelmed by Sentinel defenders and forced into a defensive position instead of an attacking one. Thel 'Vadamee was personally ordered by the [[Prophet of Truth]] to find and retrieve the "Sacred Icon" (the Covenant term for the "Index") for the Covenant. Because the Sentinels activated the Containment Shield, he could not be directly air-dropped to the Library; instead, he had to land at the periphery of the Sentinel Wall, disable the generator, and then proceed to deactivate the generator. [[Tartarus]] deployed Thel 'Vadamee in a Phantom drop ship, where Thel 'Vadamee entered the first half of the Sentinel Wall, where there was evidence of heavy Covenant casualties and abandoned infrastructure, making his way down through the facility and finally deactivating the Containment Shield.

While the first half of the Sentinel Wall had remained pristine, the second half, the one buttressing the Quarantine Zone, was infested by Flood. Sentinel resistance had been destroyed in some segments of the Flood-Infested Wall,<ref name="H2Guide" /> and in heavily infested areas, the Flood appeared to be beginning to terraform the Forerunner facility, spreading murky spores into the air. The UNSC deployed a small number of Marines to fight through the Flood-Infested Wall, but they were quickly overrun. Thel 'Vadamee was able to battle through hundreds of Flood forms that had taken up positions in the Wall, which had entered through wall-mounted conduits, although later, the Covenant deployed a team of three Special Operations Sangheili in the lower levels of the Flood-infested Wall, although they were killed as well. Finally, Thel 'Vadamee was able to make his way out of the Wall, emerging at the ground levels: the Quarantine Zone.

With the Sentinel Wall compromised by the Covenant and the Flood, and the Containment Shield lowered by Thel 'Vadamee, there was only one last line of Forerunner defenses: the [[Quarantine Zone]]. There was a significantly elevated number of Enforcers present, even automated sentinel construction factories flying over watch, and automated factories built into the landscape, each churning out Enforcers and Sentinels, further swelling their numbers even in intense combat. Here, the Flood demonstrated their unusual technical aptitude, commandeering Covenant and UNSC vehicles, possessing dozens of [[Type-32 Rapid Assault Vehicle|Ghosts]], [[Type-26 Assault Gun Carriage|Wraiths]], [[M12 Force Application Vehicle|Warthogs]], and [[M808B Main Battle Tank|Scorpions]] to do battle with the hovering Enforcers. Flood forces also boasted large numbers of Combat Forms, which appeared to infiltrate the Containment Zone by means of gaping chasms that periodically split the ground, emitting a green mist and appearing to be of indefinite depth, perhaps linkages to the Flood-infested underground.

Six Special Operations Sangheili, along with Rtas 'Vadumee, were airdropped at the periphery of the Quarantine Zone by means of Covenant drop pods, and Thel 'Vadamee joined them, rallying their efforts to secure a landing zone against large numbers of Flood attackers. Using the advantages of elevated ground, [[Type-27 Anti-Infantry Stationary Gun|Shielded Plasma Cannons]], and a [[Deployable lookout tower]], the small Special Operations contingent was able to hold an LZ for a Phantom to drop off a [[Type-46 Infantry Support Vehicle|Spectre]] and a pair of Ghosts. Meanwhile, 'Vadumee commanded his men to assist Thel in forging towards the center of the Flood-infested Quarantine Zone, so that Thel 'Vadamee could reach the Library and secure the Sacred Icon.

The Flood, despite a significant vehicular and infantry presence in the Quarantine Zone surrounding the Library, were unable to deny access to the Covenant and the UNSC. [[Miranda Keyes]] managed to slip the [[UNSC In Amber Clad|In Amber Clad]] through the Forerunner Enforcer patrols, while the Covenant launched a ground borne attack, led by Thel 'Vadamee and involving a small team of Spec Ops Sangheili, using a gondola to get to the index while attempting to control the gondola and keep it from slipping into Flood or Sentinel hands. Thel 'Vadamee took the Index from Miranda Keyes and Sergeant Johnson, who had gotten there first, but was betrayed by the Jiralhanae and knocked into the depths of the Library. Later, ''In Amber Clad'' was captured by the Flood.

===Attack On High Charity===
[[John-117]], who had arrived with [[Keyes]] and [[Avery Johnson|Johnson]], met Thel 'Vadamee in the clutches of the Gravemind, the central controlling intelligence of the Flood hive, created from the combined intellect of tens of thousands of assimilated corpses. The Gravemind had saved Thel 'Vadamee before the Sangheili could fall to his death after being hit by a burst from Tartarus' Gravity Hammer. Gravemind also managed to capture John-117 as he sank in a lake after killing the Prophet of Regret. Gravemind attempted to convince Thel 'Vadamee, against his own beliefs, and through the use of the captured [[Monitor]], [[2401 Penitent Tangent]], and the semi-revived [[Prophet of Regret]] to not activate the ring. Still unswayed, although in question of his religion following Tartarus' perceived betrayal, Thel 'Vadamee prompted the Gravemind to show him, by "stopping the key from turning," transporting John-117 to [[High Charity]], and Thel 'Vadamee to Delta Halo's [[Control Room]].

Taking advantage of the distraction that John-117's presence caused, as well as the ensuing [[Great Schism|Covenant Civil War]], Gravemind took the opportunity to deliver himself, as well as a multitude of Flood underlings, onto ''[[UNSC In Amber Clad|In Amber Clad]]'' and then performed a slipspace jump, forcing the UNSC ship inside ''[[High Charity]]'', whereupon it immediately crashed into the city's superstructure to deliver its infectious cargo, while a wave of Flood-controlled [[D77-TC Pelican|Pelican Dropships]] delivered Flood all over the city. The ill-equipped and distracted populace was no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the [[Prophet of Mercy]] falling victim to an infection form.

The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems; the whole city was becoming terraformed into a massive Flood Hive. The Gravemind regularly rebutted the words of comfort the Prophet of Truth delivered to the city’s population over the comm system, to sadistically instill more panic and unease. John-117, having successfully stayed in front of the wave of Flood running rampant in the streets, stowed away on the [[Forerunner dreadnought|Dreadnought]] that was leading the Prophet of [[Truth's fleet]] to Earth. Cortana, who had to stay behind to manage the situation should Installation 05 be fired, was left in the clutches of the Gravemind, who could potentially learn all possible human knowledge from her.

===Earth, The Ark, And The Flood's Defeat===
[[File:Infection.jpg|230px|thumb|right|UNSC Marines being infected by the Flood.]]

{{Quote|"Child of my enemy, why have you come? I offer no forgiveness... a father's sins pass to his son."|[[Gravemind]] on the level [[Cortana (level)|Cortana]]}}
{{Quote|"I am a timeless chorus. Join your voice with mine, and sing victory everlasting."|Gravemind on the level [[Floodgate]]}}
{{Quote|"Of course, you came for [[Cortana|''her'']]... We exist together now, two corpses, in one grave."|Gravemind on the level [[Cortana (level)|Cortana]]}}
At the conclusion of the [[Battle of Tsavo|Battle of Voi]], with the [[Prophet of Truth]] activating the Artifact and the Jiralhanae fleet escaping to the Ark, a Flood-infested Covenant cruiser appeared out of [[Slipstream Space|Slipspace]], having penetrated the Sangheili' blockade of High Charity and crash-landed into the city of [[Voi]], attempting to spread. In order to prevent the Flood from escaping and infecting the entire planet, [[Miranda Keyes]] ordered John-117 to detonate the cruiser's reactors to destroy the Flood. As John-117 and [[Thel 'Vadam]] fought their way to the ship, the ''[[Shadow of Intent]]'' arrived and deployed Sangheili task forces to aid surviving Marines. John-117, with the help of the Sangheili, fought their way through the Flood. Miranda later notified John-117 that the Sangheili had found Cortana inside the ship and ordered him to get Cortana in addition to destroying the ship. The group later encountered the [[Pure Form]]s for the first time, which could manipulate itself into accurate Ranged Forms or powerful Tank Forms from its Stalker Form. John-117 made his way down to the ship after the Arbiter and the remaining Sangheili stayed behind to make sure no surviving Flood would attack him. The Gravemind noticed his presence in the crashed cruiser but did not take any action against him. John-117 found the device Cortana was supposedly in and recovered it. After a Jiralhanae Combat Form attempted to attack the Spartan, Guilty Spark dropped from a Phantom and killed the Combat Form and the two were transported onto ''Shadow of Intent'' as it glassed Voi and some of its surrounding area to eliminate any remaining Flood.

With the assistance of 343 Guilty Spark, Cortana's message was repaired, revealing a solution to the Flood menace on the Ark. To Lord Hood's disapproval, Miranda Keyes led UNSC forces allied with Thel 'Vadam's and Rtas 'Vadum's Sangheili through the Portal.

On the Ark, the UNSC and the [[Covenant Separatists]] attempted to get to the Prophet of Truth, but first had to disable the shield over the Ark's citadel by attacking three towers funneling energy to the shield. John-117 led the attack on the first tower, while Thel 'Vadam led the attack on the second tower, and Sergeant Johnson assaulted the third tower. When Sergeant Johnson failed to take out the third tower, John-117 arrived there with Thel 'Vadam and a group of Sangheili. John went to the top of the tower. While attempting to lower the shield surrounding the Ark's citadel, the infested ''[[High Charity]]'' made a Slipspace jump and smashed into the Ark, releasing the Flood inside the structure and causing an infestation on the newly built Installation 08. All three factions were immediately attacked by Flood forces. Significant damage was done to the Shadow of Intent, disabling their weapon systems. The UNSC and the Covenant Separatists were forced to attack Truth on the ground. Finally, after attacking two Scarabs protecting the Citadel, the two forces managed to secure access to the Citadel. John-117, followed by Thel 'Vadam, quickly entered the Citadel. Inside, Truth, along with several Jiralhanae, was attempting to convince Sergeant Johnson to activate the rings. Miranda Keyes desperately crashed a Pelican into the Citadel to stop Truth from activating them, but was ultimately killed by Truth. Truth then forced Johnson to activate the rings. 

The Gravemind, realizing the imminent threat posed by Truth, soon sensed John-117 and Thel 'Vadam's presence in the Citadel. It quickly arranged for two Pure forms to meet John-117 and the Arbiter and explain its request. John and the Arbiter reluctantly accepted the Gravemind's help and the two attacked the Covenant forces while the Flood assisted them. Finally, they came to the end of the hall. Johnson was holding Miranda, even though it was too late, and told the two to kill Truth and stop the rings from firing. Thel 'Vadam managed to capture Truth while John-117 deactivated the rings. While Truth attempted to convince the Arbiter to help him, the Gravemind managed to infect Truth, corrupting him from the inside. Thel 'Vadam thrust his energy sword into the Prophet. With the most dangerous threat out of the way, the Gravemind then betrayed the two, having his forces in the Citadel attack them. However, Sentinels managed to subdue some of Flood, while John-117 and Thel 'Vadam fought their way through. They escaped through a small hole at the end of the hallway. There, the two saw the newly finished Installation 08. John-117 and Thel 'Vadam formulated a plan, a plan that, if it worked, would mean the end of the Flood.

John-117 needed Cortana to execute this plan, however, so he took a Banshee and flew straight to High Charity, which had crashed on the Ark. Meanwhile, Sergeant Johnson and Thel 'Vadam began evacuating their troops from the Ark in order to escape the Flood. John-117 managed to enter High Charity and fight his way through. The Gravemind was worried, but believed its Flood forms would be enough to stop the Spartan. However, John pushed ever deeper through High Charity. The Gravemind, in an attempt to stem the Spartan's progress, started torturing John-117 with telepathic messages, all while Cortana was contacting John, nearing rampancy. John-117 continued to push into High Charity, finally reaching the reactor room, where he fought through into the Sanctum of the Hierarchs. The Gravemind then realized what John was trying to do, but it was too late. John-117 rescued Cortana and managed to overload the reactors by destroying three connecting pylons. As he escaped ''High Charity'', the Gravemind started going mad. It sent Combat forms to stop the Spartan, but John fought through them all. He raced back to find Thel 'Vadam attacking Flood Pure forms. The two quickly boarded a nearby Pelican that crashed into the tower. When Cortana appeared on the holopad, John-117 fired the engines and Thel 'Vadam jumped in. As the Pelican took off, the Gravemind attempted to pull the Pelican back using one of its tentacles, but was burned by the jet wash. The Pelican raced off as High Charity collapsed behind them. Nevertheless, the Gravemind managed to escape, along with numerous Flood forms.

As John-117 raced toward the unfinished Halo, Sergeant Johnson assumed command of the UNSC frigate ''Forward Unto Dawn''. Sergeant Johnson landed the frigate as close as possible to the Control Room. Meanwhile, John-117's Pelican crashed into the snowy canyon, the place where the Control Room could be found. As the four make their way to the Control Room, the Gravemind quickly sent its forces to the Control Room. Then, it contacted John-117 once again. The Gravemind sent its forces to begin occupying the new Halo construct. However, 343 Guilty Spark arrived shortly after. He assumed command over the Sentinels and assisted John-117 and Thel 'Vadam as they climbed to the Control Room. As they went up, Sergeant Johnson supported them with his Spartan Laser. Eventually, they reached the top and Guilty Spark opened the door for them. As they hurried to the Control Room, the Gravemind began talking to John-117 again, and Cortana realized that the Gravemind was trying to rebuild its damaged form. As the four went in, the Gravemind tried to follow them with Combat forms, but was locked out. However, the Gravemind was able to regroup when 343 Guilty Spark went rampant following Johnson's attempt to activate the Halo ring prematurely, mortally wounding Johnson and locking the Arbiter out of the room. Guilty Spark then did battle with John-117, using a powerful hard-light beam that easily drained John's shields. Johnson, still alive, shot Spark with his Spartan Laser, before handing it to John while the Monitor recovered from the unexpected attack. Finally, John destroyed Guilty Spark and activated Halo using the Index that Cortana saved from the first Installation 04. The Gravemind, realizing that there was very little chance of survival, attempted to stop John and Thel 'Vadam from escaping Halo. However, the Gravemind's forces were still battling Sentinels, leaving few Combat Forms to stop the two as they took Johnson's Warthog. Despite the Flood and Sentinal forces that stood in their way, John-117 and Thel 'Vadam were still able to make it onto UNSC frigate ''Forward Unto Dawn''. As they flew into the Portal, Halo activated, killing the Gravemind and wiping out most of the surviving Flood on the Ark and on Installation 08, although it is known that at least one Flood spore survived.{{Ref/Website|URL=https://www.halowaypoint.com/en-us/news/introducing-halo-wars-2-awakening-the-nightmare|SiteName=Halo Waypoint|PageName=Introducing Halo Wars 2: Awakening the Nightmare}}

===Aftermath===
At least for the moment, the Flood were seemingly defeated. However, the reason that the Forerunners preserved some of the Flood for research on the Halo rings instead of destroying them all was that they knew that the Flood would return sooner or later, and it would be better to fight a researched enemy rather than a foreign one. After the Flood's defeat, [[SPARTAN-IV program|Spartan-IVs]] were deployed to [[Installation 03]] to hunt out possible Flood infestations.<ref>''[[Halo 4: The Essential Visual Guide]]'' – page 200</ref> Aboard ''Infinity'', Spartans did War Game simulations of Flood scenarios, including fighting in entire cities that had been infested by the Flood.<ref>''[[Halo: Spartan Assault]]'', [[Co-op Missions]]</ref>

Despite John-117 destroying High Charity, parts of it still remained intact and remained in the Ark, where the Flood began to recuperate from their disastrous defeat on Installation 08, though the Sentinels quarantined the surrounding area to ensure they could not escape. Due to a lack of fresh bodies, the Flood improvised with numerous new forms to make up for this weakness. The Flood had accumulated enough biomass to create an especially large Proto-Gravemind, which was nearing critical mass to become a Gravemind.

Seven years would pass after High Charity crashed on the Ark when [[Atriox]]'s [[The Banished|Banished]] began a war of attrition against the UNSC crew of the ''Spirit of Fire''. Needing all the resources they could find, the Jiralhanae brothers [[Voridus]] and [[Pavium]] began scavenging near High Charity. Vordius disregarded Atriox's orders and Pavium's advice to leave High Charity alone; seeking glory and wealth, he had a [[Banished Scarab]] breach the shield dome surrounding High Charity and sent three scouts to search inside, all of whom would be infected and turned into Combat Forms. <ref>https://www.youtube.com/watch?v=R1MVKdJ8X3E</ref>

The Flood immediately took advantage of the naive Jiralhanae's clumsiness and began overwhelming Banished bases and scavenging operations, converting all they could find to fuel the Flood war machine. This in turn also allowed the Proto-Gravemind to expand further onto the surface. Although the two brothers fought bravely, they knew they were fighting a losing battle. Fortunately, Voridus was able to find a partially intact Forerunner facility. The Flood deployed numerous forms to stop him but failed. Sentinels began joining the fight against the Flood. When they reached the Proto-Gravemind, the brothers were able to weaken it slightly and send [[Retriever]]s that finished it off. Atriox, who had been informed of the situation by Pavium, arrived shortly after the Proto-Gravemind was killed. He berated the two brothers, mostly Vordius, for the catastrophe that had occurred and ordered them to clean up their mess. As Atriox looked at the remains of the Proto-Gravemind, a horde of Sentinels were seen flying overhead to remove all Flood contamination.

==Biology==
[[File:Flood wall.jpg|thumb|right|A wall covered in heavy Flood biomass: evidence of advanced infestation.]] 
[[File:1246801941-Yuk.jpg|thumb|right|[[John-117]] stepping in Flood Biomass as he enters the Flood infested ''[[High Charity]]''.]]

The Flood, a virulent species of parasite, is an anomaly to all known biology, and an anomaly to regular life. This may be explained by the fact that it is an extragalactic species, originating from a different galaxy.<ref name="bestiarum" /> It is extremely adaptive, and mutates hosts through different stages of accelerated "evolution," where the body of biomatter accelerates its own evolution to the point of a massive biological breakdown, whereby the infection can first take place. The species can reproduce without host bodies, though this is described as a last-ditch effort, and only performed by pure Flood forms.

The only known pathway for Flood reproduction and survival is by the infestation and assimilation of other species. They do this through a form of cellular reproduction known as lysogenic replication, where the viral cells infect host cells with their own genetic code, mutating the host cells into viral forms.

The Flood assimilate almost any species possible to their cause, although the [[Mgalekgolo]] and [[Yanme'e]] are supposedly ignored as biomass; the former being too small and lacking a central nervous system and the latter having a chitinous body which would lack calcium. This is seen during the Flood's occupation of ''Truth and Reconciliation'', as Cortana saw them amass Sangheili and Unggoy corpses in corners of one of its hangers.

Capable of surviving extreme environments ranging from -75 to +53 degrees Celsius and even underwater (in early stages of life), the Flood can withstand the harsh environments of all known colonized planets.

It would seem that the Flood can infest any living organism, but that they prefer hosts with a large brain capacity to accelerate the formation and expansion of the Gravemind hive intelligence and the Flood intelligence as a whole. They are only as intelligent as the beings they infect; the more cunning the enemy, the more dangerous the Flood will be.

As the Flood spread and replicate, they will alter the environment to better suit their needs. This takes the form of biomass growing on any available surface, and the air being filled with spores. This appears to be part of the Flood life cycle and is done automatically, and leads to the development of a Gravemind as well as a [[Flood Hive]].

The Flood seem to thrive in moist and humid areas, which lends itself to the fact that the Quarantine Zone is bereft of organic life, and is cold and perpetually snowy, to hinder the development of the Flood.

===Neurological===
Infection forms selectively target other species that possess sentient intelligence and are of sufficient biomass, and can infest living or lightly wounded dead bodies. Infection forms have long tendrils that pierce the skin of the host and find their way to the spinal cord. They then synchronize with the host, matching frequencies with the host's nervous system and killing them almost instantly. From there, they take control of the host body, replacing whatever mind originally resided there with the ravenous and voracious psyche of the Flood, although certain higher-level cognitive abilities, such as combat techniques and technical knowledge which may be useful, are retained, scanned from the victim's brain after death. Despite this lingering of certain memories, no trace of the original mind's personality remains; only a simplistic and primal urge to assimilate or destroy other species drives the organism after infection. In isolated cases, such as when the Flood seek very specific information from an individual's mind, they utilize an alternate, undescribed method of infestation that does not kill the host immediately; this allows the Flood to "burrow" into the host's mind, eventually giving them access to the entire scope of the host's memories. This has only been specifically seen when the Flood sought information from Captain [[Jacob Keyes]], such as the location of Earth or a starship that would free them from Installation 04.<ref>'''[[Halo: The Flood]]''', page 175</ref> In addition, if the Infection Form that initiates assimilation is extremely aged or damaged somehow, the death of the host can be removed from the equation entirely; Private [[Wallace Jenkins]] suffered such a fate, remaining alive and fully aware of his situation despite the mutation of his body, even exhibiting limited control over his body at times when the Flood instincts were dormant.

If a greater Flood intelligence, such as a [[Gravemind]], is activated, then it is capable of overriding the primitive instincts of all other Flood forms, taking direct control of each and every Flood organism and making them much more dangerous, a result of being guided by a sentient mind. It is unknown what would have happened to a still-living host such as Private Jenkins when a Gravemind attempted to influence control over it, as no living hosts were ever encountered after the activation of a Gravemind. In addition to controlling the physical movements of the host body, the Gravemind can utilize the vocal cords of combat forms made from former Jiralhanae, Humans, and Sangheili to communicate, as well as the moans they use as observed on [[The Covenant (level)|The Covenant]], where the drop pod crashes into the third shield tower.

===Physiological===
While baseline neurological assimilation is the same regardless of host species, The Flood's infestation includes physiological mutilation, which is species-specific.

Higher-level species capable of combat, such as Humans, Jiralhanae, or Sangheili, are selectively infected to become Combat Forms. Combat Forms retain the general profile of their original species, although they are significantly mutated. Organ systems are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hitbox-selective incapacitation impossible; organ functions are decentralized, thus, decapitation does not kill a combat form. Furthermore, highly area-specific munitions, such as sniper rifle projectiles, are completely ineffective, as picking off selective areas of a combat form leads to no wide-scale physiological impediments, and the projectile will punch through the decayed flesh rapidly, exiting through the other side of the combat form without consequence.

Combat forms also generate tentacles very quickly without regard for the skeletal structure of the host, with tentacles protruding from the combat form at odd locations, typically localized to the chest axial regions, such as the neck, armpits, or torso. A single arm is oftentimes mutated to a claw like or tentacle-like structure that splits into multiple angular, hand-like structures, replacing the hand's usage, though it will still be there. The musculature of the combat form dramatically changes as well, and the form is given prodigious arm and leg strength, allowing for heavy blows with its arm-like structures and for incredible speed and jumping height with its legs. The Flood are also prodigious climbers, able to scamper on walls with little hindrance to the organism . Metabolic requirements, however, appear to be extreme on the organism: blood vessels become extremely prominent on the surface of the Combat Form, with thick and distinct blood-colored vessels spawning all across the creature's surface, most prominently so on the head.

===Countermeasures===
[[File:Flood Eat.jpg|thumb|A Marine being attacked by an Infection Form.]]
There is no confirmed way to effectively prevent a Flood infection once an Infection Form infects a host, and there is no way to reverse the effects of flood infection. However, there are known medical conditions that scramble the neural frequencies of the host (apparently without major negative manifestations in the host) which subsequently prevents flood infection. For example, flood infection is impossible in a host that has contracted "[[Boren's Syndrome]]," a neurological condition caused by radiological exposure that creates electrical anomalies in the nervous system. Boren's Syndrome is a common side effect of the biologically enhancing drugs used in the [[ORION Project]], the precursor to the [[SPARTAN-II Program|SPARTAN Program]] . It has been suggested that the scrambling of neural frequencies caused by Boren's Syndrome and similar conditions renders an Infection Form incapable of tapping into the host's nervous system to kill and infect him or her. The Infection Form is therefore unable to enact any large-scale physiological mutation; however, some Flood DNA is still spliced with the host's DNA, apparently imparting the host with enhanced regenerative capabilities. However, this is only speculation and has yet to be independently verified. [[Avery Johnson|Avery J. Johnson]], a Boren's Syndrome patient, was the only survivor after an outbreak in the containment facility on Installation 04 where The Flood attacked two squads of UNSC Marines. However, the Boren's Syndrome was a story invented to cover his past in the [[ORION Project]] and the side-effects of his (experimental) enhancements, so the validity of each as a countermeasure against Flood infection is questionable. In the novel [[Halo: The Flood]], [[Cortana]] uses an electrical charge which she generates from [[John-117]]'s suit to destroy an infection form that was attempting to infect him.

While Boren's Syndrome, if left untreated, can leave the patient immune to the Flood infection, the disease will prove fatal within a couple of years. Halsey once stated that there is a one in a billion chance that research, from someone so inflicted with Boren's Syndrome in a way that they cannot be infected, could be carried on to produce a cure.

Since the Flood is a parasitic life form, they depend on other organisms for survival. Thus, the only logical foolproof way to eliminate them permanently is to take away their food sources. The Forerunners attempted this by bombing large population centers, but this was seen as one of their last resorts and as such was performed too late in the war to make much difference. The absolute last resort was the use of the Halo installations, which effectively destroyed all life in the galaxy and starved the Flood into extinction. However, the Forerunners' plan included some measures to preserve sentient life and return them to their home planets, allowing the Flood's "food source" to persist. Unfortunately, at the same time, specimens of Flood spore cells were kept for analysis on the Halo Installations, allowing the threat of the Flood to be merely forestalled, not eradicated.

Environmental stimuli seem to have an effect on Flood metabolism. Halo installations typically artificially incorporate low temperature into environments suffering Flood outbreak and to create natural obstacles to Flood progress, regarded as [[Quarantine Zone|Quarantine zones]].<ref>'''Halo 2''', level ''[[Quarantine Zone (level)|Quarantine Zone]]''</ref> [[Cold Storage|At least one]] Flood Research Facility used low temperatures to keep its specimens inactive and dormant as it studied them.<ref>'''[[Halo 3]]''', multiplayer level ''Cold Storage''</ref> This may be the reasoning behind keeping Installation [[Control Room]]s in low degree weather, impeding their occupation by the Flood. The Flood are also vulnerable to temperatures on the other end of the spectrum. Plasma weapons will burn through them, as well as [[Sentinel Beam]]s, which were specially designed to counter the Flood, indicating that the Forerunners had determined energy weapons to be the best 'basic countermeasure' available to fight the Flood. It may be a side-effect of the Flood's conversion of the body into biomass, breaking down cellular integrity and increasing flammability.

There was a Forerunner belief that during their final years, Prehistoric Humanity had found a cure to the Flood. In spite of massive Forerunner efforts to recover the data, it was either lost or destroyed when, in anticipation of their inevitable defeat by the Ur-Didact's Fleet, humanity began systematically destroying their own technology.

The lost cure altered the DNA of inoculated humans in a manner that upon infection, the altered DNA would act as a corruptive catalyst when bonded with the aggressive Flood mutagen; resulting in the physiological degradation of the Flood's neural inhibitions, to the extent that when this new Flood entity arose, it would consume ''all'' biological entities - including other flood (both the traditional and Human-mutated specimens), which would in turn awaken as more altered organisms, thus self-perpetuating the disease into proportions sufficient to cause genocide through mass cannibalistic attrition.

Humanity injected one third of their population with this mutagen and then placed themselves in the Flood's path, ensuring the Flood would assimilate the disease. The plan worked, and the Flood were successfully stopped.

However, it was later revealed through discoveries by the Forerunner Bornstellar that there never was a cure; that it was humanity's displayed acts of how far they would be willing to go to survive that prompted the Flood's leaders to spare them. This made humanity the first and only species to pass the Precursor's cryptic 'test'.

===Developmental Stages===
The Flood go through four distinct developmental changes.
#[[Feral Stage|'''Feral''']] stage is the lowest form: communicating via pheromones, and have the natural instinct to harvest enough calcium to establish a viable Gravemind. (Seen in ''[[Halo: Combat Evolved]]'' and ''[[Halo Wars]]'')
#[[Coordinated Stage|'''Coordinated''']] stage is when the Flood becomes truly dangerous; they are now controlled by the Gravemind that was created in the first stage. (Seen in ''[[Halo 2]]'')
#[[Interstellar Stage|'''Interstellar''']] stage allows the Flood to take control of most technology and spread throughout the galaxy to infect more hosts. (Seen in ''[[Halo 3]]'')
#[[Intergalactic Stage|'''Intergalactic''']]<ref name="bestiarum">'''[[Bestiarum]]'''</ref> is the last known stage the Flood enters, theoretically, in which it utilizes all captured technology to depart to uninfected galaxies to further replicate.

===Flood Forms===
[[File:H4hd 9.jpg|thumb|250px|right|The Flood as they appear in Halo 4]]
The most basic form of Flood evolution is the infection form, unique in its distinctive spherical shape. The infection form is used to infect organisms of other species, physically adhering to their surfaces and inserting tentacles, forcibly "hacking" into its host's nervous system, completely changing its psychology to that of the Flood's, and eventually physiologically mutating it into one of the various parasitic Flood forms of whatever the victim may be. The Sangheili combat form may use the shield in the armor of the host. The Tank form is about the size of a Hunter, and can release Infection forms from its body. It can mutate into two different forms: Stalker form, and Ranged form.

{{Flood}}

==Combat Behavior==
[[File:Floodambush.jpg|thumb|right|The Flood emphasizes "numerical superiority" in engagements and also aggressive swarming tactics.]]
While possessing an unusual sentient intelligence, with Combat Forms still retaining extremely high-level cognitive functions such as vehicular operation, traits such as dexterity and complex battlefield tactics are notably lacking in the Flood, especially in their psychological and combat behavior. While the Gravemind coordinates Flood strategy on a galactic scale, small encounters are not coordinated between individual Flood warriors.

The Flood's strategy is simple: They throw themselves at potential hosts in huge numbers with any and all weapons available. Flood tactics are based primarily on asymmetrical warfare: the Flood uses all of its different forms to kill any enemy it sees. Infection forms swarm across the battlefield in waves, reanimating any corpses they can find, while Combat forms attack and kill any hosts in sight. Pure forms, if present, provide support to Combat forms in a number of ways: Stalker forms move to strategic positions and mutate into either Tank or Ranged forms, which can destroy enemy vehicles or fortifications or provide suppressing fire from afar.

The Flood have a strong preference for close-range combat: Combat forms possess abnormally strong melee attacks by whipping their tentacles, capable of lowering a Spartan-II Mjolnir armor’s shields to half power with a single hit. Carrier forms cause tremendous damage at close ranges, and their explosions are extremely devastating to friends and foes alike, with the added benefit of disseminating infection forms in the immediate vicinity. Infection forms are capable of attacking only at point-blank ranges, exploding in damaging bursts when meeting an opponent's personal [[energy shielding|energy shields]] or instantly grabbing hold of an unshielded enemy organism and infecting it. Combat forms can also use firearms, and have only ever been seen clutching them in one hand (regardless of whether it is one-handed or two-handed). However, combat forms struggle with ranged weaponry: it is believed that the neurological ramifications of Flood infection significantly decrease accuracy, as well as weapon recoil, if held in only one hand. The Flood are, on the other hand, very effective with heavy weaponry such as the [[M19 SSM Rocket Launcher|M19 Rocket Launcher]] or the [[M90 Close Assault Weapon System|M90 Shotgun]], and are curiously unskilled with close-quarters weapons such as the [[Type-1 Energy Weapon/Sword|Energy Sword]]. However, it is still possible that a Flood with an Energy Sword can kill a fully shielded Sangheili or Spartan in one blow.

As mentioned previously, the Flood demonstrates a curious higher intelligence despite their poor combat aptitude and coordination. The Flood collectively learn any general information a species possess, even if only one member of that species is assimilated. For example, during the ill-fated raid on ''[[Infinite Succor]]'', after only one Sangheili was infected, the Flood could open any doors and access any system on the ship immediately afterwards, demonstrating that the Flood can share collective knowledge across their species. Furthermore, when the Flood assimilated the [[Minister of Etiology]], the Flood reflected the high-level knowledge of him through its individual organisms to Special Operations Commander Rtas 'Vadumee, mocking him with the words, "''the Forerunner could not defeat us...what chance you?''" The Flood, however, reflect unusual knowledge in the individual psychologies of its individual species in processes facilitating increased infestation: The Flood were sighted gathering bodies in the corners of the Covenant ship ''Truth and Reconciliation'', perhaps making it easier for Infection Forms to find hosts to reanimate. The Flood were also gathering body parts for a new [[Proto-Gravemind|Brain Form]] aboard ''Infinite Succor''.

[[File:Catch.jpg|thumb|right|Typical Flood combat behavior: ravenously targeting the nearest enemy, with infection forms running ahead of combat and carrier forms.]]
Flood Combat forms have been sighted crudely operating [[Type-32 Rapid Assault Vehicle|Ghost]]s, [[M12 Force Application Vehicle|Warthogs]], [[Type-26 Assault Gun Carriage|Wraiths]], and [[Scorpion]]s in ground operations, although with slow deliberation and clumsy driving and accuracy. The assimilated crewmen of {{UNSCShip|In Amber Clad}} were also able to initiate a Slipspace micro-jump for the frigate and perform rudimentary maneuvering of ''In Amber Clad'', navigating the frigate from the atmosphere of ''[[Installation 05|Delta Halo]]'' to ''High Charity'' and then crashing the vessel into a tower, releasing Pelican dropships to rain down upon the capital.

The Flood's millennia of existence has made the assimilation process extremely brief: hosts killed in a short firefight can be reanimated within seconds to minutes by an Infection form, and live creatures can be mutated into fully developed Combat forms within the same time frame. Sometimes the rate of the mutations can vary in extremely rare cases: when Rtas 'Vadumee battled his comrade-turned-Combat form Bero 'Kusovai aboard the ''Infinite Succor'', 'Kusovai continued to mutate throughout the duration of the battle at a restrained and constant rate. Though never seen within the story, it can be assumed Infection forms were involved in infecting a victim. Before being killed to prevent him becoming a Flood Combat form, one of the Sangheili accompanying Rtas 'Vadumee had a wound in his chest that was consistent with the entry point for an Infection form.

The Prophet of Truth, in his final moments, was infected by the Flood and began to mutate, but the rate and nature of his transformation was not consistent with the usual process when a sentient being is taken by a Flood Infection form. It is likely that this was influenced by the nearby Gravemind, who was either controlling the Infection form's activity, or controlling the mutation of the Flood spores within Truth's body.

==Notable Victims==
{{col-begin}}
{{col-2}}
*[[Minister of Etiology]]
*[[Wallace Jenkins]]
*[[Captain (Navy)|Captain]] [[Jacob Keyes]]
*Sub-Commander [[Bero 'Kusovai]]
*[[Private]] [[Manuel Mendoza]]
*[[Lance Corporal]] [[Eugene Yate]]
{{col-2}}
*[[Prophet of Mercy]]
*[[Private]] [[Riley]]
*[[Prophet of Regret]]
*[[Prophet of Truth]]
*[[Bok]]
{{col-end}}

==Tactics==
===''Halo: Combat Evolved''===
It is best to use the [[MA5B Individual Combat Weapon System|MA5B Assault Rifle]], Shotgun, or [[M6D Personal Defense Weapon System|M6D Magnum]] against the Flood. The Magnum is capable of killing a combat form with a single shot to the chest, but is generally not recommended for mass engagement, as its medium rate of fire and small magazine size can lead the player into a position where they are surrounded by Combat Forms and need to reload. Therefore, it is best to apply it only in long-range combat. Furthermore, Combat forms have a tendency to revive after being shot once with a pistol. The [[Type-25 Directed Energy Rifle|Plasma Rifle]] is also a good weapon of choice to use (if the assault rifle/shotgun are unavailable) as its high rate of fire can quickly destroy Infection Forms while still being powerful enough to destroy Combat Forms.

The Sniper Rifle is ineffective against Combat forms due to the Flood's organ-function decentralization. The Shotgun, on the other hand, is one of the best choices against Flood Forms, as it usually finishes them off in one or two shots, and prevents Combat Forms from suddenly rising up again.

The Assault Rifle, an all-round effective weapon with large spread range, is superb against masses of Combat forms and Infection forms. Never bother to melee Combat Forms or Carrier Forms, as it is practically useless and very time consuming. For Infection forms however, a single melee can take down four to five of them.

All in all, the most practical option is to have a shotgun for close-up/quick work, and a Pistol or Assault Rifle for longer range. The pistol is most useful for the level [[Two Betrayals]], while the Assault Rifle/Plasma Rifle is preferred on [[The Library]] due to the levels cramped nature.

It is possible to shoot the arms off of Combat Forms (alive or dead). Dismembered Combat Forms pose no threat. This works well on the level [[Keyes]].

===''Halo 2''===
Dual wielding [[M7/Caseless Submachine Gun|SMGs]] and other dual-wielding weapons have proven to be very effective, as well as the [[Sentinel Beam]], [[M90 Close Assault Weapon System#M90 Mk II|Shotgun]] and [[Type-1 Energy Weapon/Sword|Energy Sword]]. [[Type-25 Directed Energy Pistol|Plasma Pistols]], however, are the least useful. Dual-wielding [[Type-25 Directed Energy Rifle|Plasma Rifles]] and aiming at the arms of Flood Combat forms is more than enough to rip them apart easily. The arms will come off quickly and therefore leave the Combat Form unarmed, which will then explode after a few seconds. Shooting the Infection Form where it has burrowed itself in the host will also kill the Combat Form instantly, assuming that it has no shields.

The SMG is an excellent weapon against the Flood, as long as ammo is in large supply. It is often a good idea to use the [[Type-25 Grenade Launcher|Jiralhanae Shot]]'s melee, as it will have the same effect as an Energy sword on Combat Forms; it will destroy the body and prevent Infection Forms from reanimating it. For one of the best combos, use dual [[M7/Caseless Submachine Gun|SMGs]] for Combat and Infection forms, and keep a [[M90 Close Assault Weapon System#M90 Mk II|Shotgun]] in reserve. Another highly effective combo is to carry a Battle Rifle for engaging the Flood at long ranges and carrying a [[M90 Close Assault Weapon System#M90 Mk II|Shotgun]] for when you don't have enough time to be precise with your aim. A good strategy on The Oracle is to dual wield a pair of [[Type-33 Guided Munitions Launcher|Needlers]]; you can usually do this without running out of ammo. Also, don't let the Infection forms touch you if you have no shields; otherwise, they will kill you instantly.

On a side note, weapons such as the Sniper Rifle, Beam Rifle, Battle Rifle, Covenant Carbine (and other headshot-capable weapons) are efficient weapons for killing combat forms, provided you have a quick trigger finger, good aim, and the [[Grunt Birthday Party Skull]] activated.

===''Halo 3''===
The [[Type-3 Antipersonnel/Antimatériel Incendiary grenade|Incendiary grenade]] instantly destroys all Flood Pure forms and is the one true weakness of all pure Flood forms, as well as its Support Weapon counterpart the [[M7057/Defoliant Projector|Flamethrower]]. Also, all forms can be instantly destroyed with any form of melee attack, with the exception of the Pure Forms, which cannot be revived anyway when killed. The method of killing a Combat Form in one shot in Halo 2 is still viable in Halo 3. However, the corpse will be susceptible to infection by another Infection Form unless the body destroyed.

Plasma weapons are very good to have when fighting the Flood, since the plasma burns the bodies. The [[Sentinel Beam]] is also very accurate when shooting the Flood, seeing as it also burns their bodies, killing them faster. Some players find the [[Type-25 Grenade Launcher|Brute Shot]] to be an effective Flood-busting weapon, as the grenades will destroy a Combat Form, knock a Ranged Form off a wall, and pop a group of Infection Forms and a Carrier Form easily. The blade attachment can kill any Pure Form easily and instantly. [[Type-1 Energy Weapon/Sword|Energy Swords]] are also effective as they are more accurate, lunge faster and can kill both Combat and Pure Forms in one lunge.

===''Halo Wars''===
Their tactics on ''Halo Wars'' are very similar to those they perform on the Halo Trilogy; they can obviously infect Humans, Sangheili, Jiralhanae and now even Kig-Yar and Unggoy with their Infection Forms, and usually attempt direct attacks with large groups. They possess large tentacle-like Flood forms and large pods that can pin in and attack vehicles, in the case of the Elephants they capture, which is an objective to rescue in the [[anders' Signal|first level where they are encountered]] in-game. The Flood establish bases by capturing and spreading an infestation on enemy bases.<ref>'''[[Halo Wars]]''', level ''Anders' Signal''</ref>

One of the newer, scarier aspects of the Flood developed for Halo Wars is their ability to transform sentient life with "Flood Spores"<br />Certain units and Flood "Structures" when destroyed release what appears to be an intelligent cloud of green spores which swarm across nearby units.<br />Once any unit has received contact for a long enough period of time (A matter of seconds in some cases) the unit is considered lost and becomes a Combat form.<br />Tactically this completely changes the way in which a person has to think, rendering all infantry (save Spartans) completely useless in combat.<br />

The Flood have been confirmed by Ensemble to have been made overpowering to both UNSC and Covenant forces to keep their appearance terrifying. Early in development, Ensemble considered making the Flood a playable race, but this idea never made it past the concept stage. According to lead designer Graeme Devine, the main issue was that in order to balance the Flood with the UNSC and Covenant, the Flood would have to have been similar to StarCraft's Zerg, which did not mesh with the Flood's role in the games: to be "the single scariest thing in the galaxy."

===''Halo: Spartan Assault''===
In the [[Xbox One]] and [[Xbox 360]] versions of [[Halo: Spartan Assault]], the flood employ very similar tactics to their ones in the Halo trilogy. Infection forms will simply run at the player in an attempt to swarm them, while Combat forms attack using weapons. However, Combat forms seem to have far more intelligence in this iteration and [[melee]] attack far more rarely, opting to use their ranged weapons and grenades instead.

It should also be noted that a yellow Infection form is released upon the death of a human combat form and a large (around 6) amount are released on the death a [[Flood Combat Carrier Form|combat carrier form]]. The yellow Infection forms can latch themselves onto the back of the player, draining their health until they are dead.

==Changes==
===Changes from ''Halo: Combat Evolved'' to ''Halo 2''===
*The Flood can make screeches and roars, which were not present in the first game, although they still continue to make low growls upon sighting [[John-117]].
*Skin is more textured, giving the Flood a more realistic and disgusting look. The faces of the human forms are also more readily recognizable.
*If a Combat Form has both arms shot off, it will walk slowly then explode, releasing an Infection Form.
*Rather than having to "gather bodies," the Flood can seemingly infect hosts over the course of a few minutes or even a few seconds.
*Rather than corpses re-animating themselves, an [[Flood Infection Form|Infection Form]] will enter the body, then re-animate it.
*If a player is jumped by an Infection Form while their shields are down, they will die instantly. In ''Halo: Combat Evolved'', it would simply take a bar of health away.
*Some Combat Forms derived from Sangheili still have functional energy shields.
*The Gravemind is introduced, making the Flood a more significant faction in the series.
*The Flood can use vehicles, albeit clumsily.
*The melee attack does little to no damage against the Flood in ''[[Halo: Combat Evolved]]''. It does a bit more damage in ''[[Halo 2]], ''and can take 1-2 hits to destroy an entire Combat Form from the bayonette of the [[Type-25 Grenade Launcher|Brute Shot]].
*The Flood appears to be "smarter" when in combat, preferring to stay back and fire on the player rather than "rush" them. This could be because of a Gravemind or merely the AI code got updated.
*In ''Halo: Combat Evolved'' a [[Flood Combat Form|Combat Form]] would walk slowly when shooting at the player. When losing the arm that shoots, it would go "berserk": charging at the player, madly flailing its remaining arm. In ''Halo 2'', the combat forms can "decide" whether to use a melee attack or to shoot at the player.
*If a player is wielding an [[Type-1 Energy Weapon/Sword|Energy Sword]] or [[M90 Close Assault Weapon System|Shotgun]], a Combat Form may retreat. They will not do this if a player is wielding other weapons.
*A [[Flood Juggernaut (Halo 2 Cut Material)|Juggernaut]] Form was put in, but was not placed in any accessible area of the game, although it was kept on the disc.
*The burst radius from killing an Infection Form is significantly decreased to prevent the player from simply killing one Infection Form to dispatch a whole swarm.
*Flood Combat Forms do not "feign death" anymore.
*Flood Combat Forms can board the players vehicle, and then will proceed to melee the player.
*Infection Forms appear less textured and detailed than in ''Halo: CE''.

===Changes from ''Halo 2'' to ''Halo 3''===
*[[Jiralhanae]] may now be infected and transformed into Combat Forms.
*Melee attacks and plasma weapons were nearly useless in ''Halo: Combat Evolved'' and ''Halo 2'', but now can be used effectively against the Flood.
*Also, any weapon can be used to dismember Combat Forms, either by melee or shooting. A realistic touch, considering the Flood's weak, rotten flesh. Thus, all Combat Forms are significantly easier to kill.
*The reanimation process is now visible in gameplay; Infection Forms can mutate humans, unshielded Sangheili and Jiralhanae into Combat Forms in a matter of seconds. In Halo: Combat Evolved and Halo 2, infection forms would simply kill the victim, and be unable to infect the body in front of the player.
*Pure forms are introduced, consisting of the [[Flood Tank Form|Tank Form]], the [[Flood Stalker Form|Stalker Form]], and the [[Flood Ranged Form|Ranged Form]]. In addition, the [[Flood growth pods]] and [[Flood dispersal pod]]s can be first seen in the fourth and seventh Playable Campaign missions "Floodgate" and "The Covenant" respectively.
*Gravemind has the ability to speak through his minions during combat. Although it is unknown whether he had the ability to do this previously but just didn't choose to.
*Gravemind also seems to have the ability to speak directly to John-117, as demonstrated in the level [[Cortana (level)|Cortana]]. It is unknown if this applies to all humans or not.
*Combat Forms wield weapons far less than in both ''Halo: Combat Evolved'' and ''[[Halo 2]]''.
*Carrier Forms release significantly more Infection Forms when they explode. The explosion does not push the player away much as in ''Halo: CE'' and ''Halo 2''. In addition, Carrier Forms are significantly more difficult to kill with projectile weapons, but they are easily destroyed by plasma weapons.
*If an Infection Form hits you while your shield is down it will not only make a crunching noise, but also it will most likely kill you on higher difficulties.
*Running over Infection Forms in a vehicle can result in a bumpy ride. The Infection Form that was hit by the vehicle will not pop.
*Carrier Forms will blow up and no infection forms will come out if it is hit by an incendiary weapon, a Plasma grenade, or Spike grenade.

===Changes from ''Halo 3'' to ''Halo Wars''===
*In Sangheili forms at least, Flood Combat Forms no longer push back the heads of their victims. In fact, the Sangheili's head now moves with the form as if the Flood actually use it now. In addition, Flood Combat Forms have more tentacles than before, and resemble spikes rather than flexible appendages.
*Infection forms are somewhat "chubbier" if compared to the trilogy version.
*Unggoy and Kig-Yar are now able to be converted to Combat Forms.
*The Infection Forms don't "explode" when they are shot in the cinematics.
*There are many new forms of Flood introduced.
*Carrier Forms don't always explode upon death.

===Changes from ''Halo Wars'' to ''Halo: Spartan Assault''===
*When [[Flood Combat Form|combat forms]] are killed, they release an [[infection form]].
*[[Flood Combat Form#Human Form|Human combat forms]] and [[Flood Combat Carrier Form|combat carrier forms]] can throw grenades.
*A new yellow infection form is added which has the ability to latch onto the back of [[SPARTAN-IV program|Spartans]] and deplete their shields and health until death if not removed.

==Trivia==
*The Flood's presence on the Halo rings and other Forerunner installations is due to the Forerunner's desire to study them and preserve them as a species.<ref>'''[[Terminals - Halo: Combat Evolved Anniversary]]'''</ref>
*"Inferi" is Latin for "the dead" and "Redivivus" means "renovated, reconstructed, rebuilt or recycled."
*Every time a [[Flood Combat Form|Combat Form]] (not any of the Pure Forms) dies, the infection form in the chest will disappear.
*There is a species of fungi that is frighteningly similar to the Flood, the ''Cordyceps unilateralis''. They attack carpenter ants and take over their neural system in order to make the ants go to high places. By the time it gets there, it's been rotted from the inside out like a Flood host and it releases spores from that vantage point.<ref>[http://neurophilosophy.wordpress.com/2006/11/20/brainwashed-by-a-parasite/ '''Neaurophilosophy''' - ''Brainwashed By a Parasite'']</ref>
*In a PC World article, the Flood was voted as #31 of the top 47 most diabolical video game villains of all time<ref>[http://www.pcworld.idg.com.au/index.php/id;338891934;pp;1 '''PC World''' - ''The 47 Most Diabolical Game Villains of All Time'']</ref> and #45 on IGN's Top 100 Videogame Villains list.
*The Flood were inspired by Christopher Rowley's ''[http://www.christopherrowley.net/novels/VangSeries.html The Vang]''.<ref>[http://www.gamespot.com/xbox360/action/halo3/news.html?sid=6162009&mode=previews&msg_sort=1&cpage=11 '''Gamespot''' - ''Halo - Q&A'']</ref>
*In ''[[Halo: The Flood]]'', John-117 was attacked by an Infection Form while his shields were low. The Infection Form had slit the under parts of his armor, exposing his neck, and almost tapped his spinal cord. If not for the quick use of an electrical shock by Cortana to kill the form, John-117 would have been infected.
*The name "The Flood" is a reference to the biblical story of the Great Flood in which God destroyed all life except for that which was carried by Noah's Ark.
*In ''Halo 2'', Combat Forms will go berserk if you kill their allies near them. They drop their weapon and charge at the nearest enemy.
*[[Mgalekgolo]] are immune to Flood infection.
*Sergeant Avery J. Johnson is the only known human to survive a direct assault by an Infection Form without aid. This is possibly due to the fact he is a member of ORION and his genetic makeup is different from that of a regular human being.
*[[The Mona Lisa]], a short story found in [[Halo Evolutions]], demonstrates that the Flood can also infect an organism in other ways than with an infection form. One Marine became almost instantly infected after being merely slashed in the back by a Combat form. The same infected Marine later bit the face off of another Marine in her squad, infecting him as well. This form of infection resembles the modern trend of zombies infecting normal people by biting them.
**Considering writers Tessa Kum and Jeff VanderMeer were not given access to Halo's story bible when writing ''The Mona Lisa'', it's difficult to say if this is considered a canon form of Flood Infection.
*''[[Halo 4]]'' is the first game where it is possible to play as the Flood.
*If an entire ecosystem of sufficient biomass is infected, the entire planet becomes a [[Keymind]].<ref>''[[Halo: Silentium]]'', page ??</ref>

==Gallery==
<gallery widths="150" spacing="small">
File:HCE-Flood elite.jpg|Concept art of an infected Sangheili from ''Combat Evolved''.
File:HCE-Flood marine.jpg|Concept of an infected Marine from ''Combat Evolved''.
Human combat form.jpg|Human Combat Form
Flood infection form.1.jpg|An Infection Form
File:H2A Gravemind.png|The Gravemind
Flood_carrier.jpg|Flood carrier form
09b-Flood1.png|Showing the various flood forms and their size differences.
Mega-Bloks-Halo-Flood-Pod-Elite-Combat-Form-13841435-5.jpeg|A mega bloks Sangheili combat form.
POAEngineRoom.jpeg|Sentinels fighting Flood Infection Forms.
H4-FloodCombatForm-Spartan.png|The infected Spartan from the ''Halo 4'' Flood mode.
Flood-1-.jpg|A group of Sangheili-Combat Forms with their melee arms severed.
Flood Tank Form.png|Flood pure tank form
Flood stalk.png|A Flood stalk
Flood Stalker.jpg|A Flood stalker
</gallery>

==Appearances==
*''[[Halo: Combat Evolved]]'' {{1st}}
**''[[Halo: Combat Evolved Anniversary]]''
*''[[Halo 2]]''
** ''[[Halo 2: Anniversary]]''
*''[[Halo 3]]''
*''[[Halo 4]]'' {{C|Simulation only}}
*''[[Halo Wars]]''
*''[[Halo Wars 2]]''
*''[[Halo: The Flood]]''
*''[[Halo: First Strike]]'' {{Mo}}
*''[[Halo Legends]]''
*''[https://halo.fandom.com/wiki/Halo:_Evolutions_-_Essential_Tales_of_the_Halo_Universe Halo: Evolutions - Essential Tales of The Halo Universe] ''
**''[https://halo.fandom.com/wiki/The_Mona_Lisa The Mona Lisa] ''
*''[[Halo: Cryptum]]''
*''[[Halo: Primordium]]''
*''[[Halo: Silentium]]''
*''[[Halo: Spartan Assault]]''
*''[[Halo: Escalation]]''
*''[[Halo Infinite]]

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[[Category:Halo: The Flood]]
[[Category:Halo: Evolutions]]
[[Category:Halo: Primordium]]
[[Category:Halo: Silentium]]
[[Category:Human-Covenant war]]
[[Category:Forerunner-Flood War]]
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