The Big D was the greatest catastrophe mankind ever encountered, a genetically engeneered virus made by the american military for biological warfare, which would kill everyone "beyond the age of innocence"; everyone beyond puberty. In the year 2006, the virus escaped from secret testing sites in India and the Middle East and quickly spread all over the globe, leaving billions of people dead and killing almost all of the adult population, with only the kids left to rebuild. The only civilian adults who survived formed the order of the Brothers of the Apocalypse and wait in a secluded monastery for the second return of the plague.
Spreading of the virus
The Big Death has a very short incubation time (less than 24 hours) and is most likely to spread through the air, although nobody ever quite found out, since most scientists involved in its creation were killed by it. The ones remaining hide out at the US Army base Valhalla Sector and try to find a cure since they fear that their creation may come back. The first vaccine they found turned out to be a even more aggressive plague, killing everyone, not only the adults, but moving much slower. Teams of soldiers with anti-toxic armor and flame throwers, sent by Valhalla sector, track down places where they had been testing this first vaccine and burn them to the ground.
In reference to the post-apocalyptic Showtime series Jeremiah.