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". This is not to be confused with the Great Cataclysm - the event that caused the continent of Atlantis to sink beneath the waves.
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.
In the Countdown to Final Crisis, Karate Kid is stricken with the mysterious Morticoccus virus that is killing him and his fellow Legionnaire, Una, travels with him to find a cure. They end up on an alternate Earth where he dies and releases the virus all over the world. People begin changing into animals and the world is destroyed, essentially creating the world of Kamandi on the alternate world.