The Great Disaster was a cataclysmic event of global proportions which eradicated civilized society on Earth-AD. In fact, the planetary designation, Earth-AD actually stands for "After Disaster".
History
Earth-AD
The story of Earth-AD begins in the future of a parallel dimension. A group of peaceful, humanoid aliens arrived on Earth with the foreknowledge of a coming Great Disaster. Hoping to prevent the Disaster from ever taking place, they established a united defense force known as the Global Peace Agency. One of the GPA's scientists, a human named Professor Myron Forest created a powerful satellite computer system called Brother Eye to help oversee over Earth's affairs. He also became provincial in transforming an army stockroom clerk named Buddy Blank into a one-man army corps. Using the name O.M.A.C., Buddy worked in the interests of the Global Peace Agency in fighting Earth's enemies.
For all of OMAC's efforts, the Great Disaster could not be averted. Very little is known as to the true causes of the Disaster, but what scant evidence exists points towards a mysterious energy wall known only as the Vortex. Earthquakes and tidal waves erupted all across the globe and the nations of Earth found themselves at the mercy of nuclear Armageddon. Shortly before the final bastions of humanity were to fall, a scientist named Doctor Michael Grant invented a mutagenic chemical known as Cortexin. Although he would not survive long enough to witness the fruits of his labor, the Cortexin drug would succeed in carving out its own legacy in the wake of global annihilation.
Earth-51
The Monitors of the Multiverse and Raymond Palmer of New Earth had foreseen a Great Disaster that would befell an Earth. Ray Palmer discovered this after venturing into Earth-51 and learned from notes written by this universe's Ray Palmer of the Morticoccus virus, which potentially precipitated the viral outbreak of catastrophic proportions. In a turn of events, Karate Kid of the Legion of Super-Heroes was infected with Morticoccus virus and became patient zero for the Great Disaster.
After Earth-51 was destroyed and recreated, Monitor Solomon deliberately let the Great Disaster happen in this universe by sending the infected Karate Kid and his friends there. Ray Palmer and Cadmus' scientists failed to prevent the outbreak, as humans de-evolved to develop animal features, and animals developed human features. As a result, civilization fell as humanity panicked and nations accused one another for causing the outbreak, causing a nuclear holocaust across the Earth. Furthermore, the universe fell into the same circumstances as Earth when Earth-51's Green Lantern Hal Jordan attempted to seek help from alien worlds, inadvertently spreading the Morticoccus virus.
Prime Earth
The Great Disaster occurred in one possible future of Prime Earth. As on Earth-AD the boy known as Kamandi was the last sentient human on Earth. Kamandi later travelled back in time to the present and assumed the identity of "Kingsley Jones", a leading member of Checkmate.
Malcolm Merlyn posing as an elderly version of Green Arrow from the future enlisted Amanda Waller's help to supposedly avert the Disaster. He claimed that Team Arrow were responsible for the Disaster and they therefore needed to prevent Team Arrow from ever fully uniting. They secretly installed teleportation devices in Team Arrow's necks, which randomly teleported them through time and space if enough of them ever managed to group together in one place. In reality, this was all an elaborate scheme to make Green Arrow miserable by keeping him from his family, and possibly even cause the Disaster to occur.[1][2]