Dontar contracted an unidentified time pirate to pull off a heist to get the TX-90 out of 2235. However, the time pirate encountered difficulties when another person, Alec, was infected. Alec apparently killed many people but the pirate escaped into the temporal zone with the infection. When he crash landed in Mississippi of 1863, he infected some Confederate soldiers with the disease, which quickly spread.
The resulting infection from the pirate caused a time aberration so the Legends investigated it. Infected soldiers prevented a vital dispatch from Henry Scott from being delivered to General Ulysses S. Grant, thereby allowing the Confederates to win the American Civil War. Ray Palmer and Martin Stein were able to develop a vaccine to the bioweapon which the Legends later distributed to those infected fixing the aberration.
The bioweapon is a virus spread through biting and causes coughing, sweating, and bloodied eyes. It also allows those who have died to reanimate into a zombified state, spreading the disease further. Zombified hosts show signs of growling, pale skin, extreme aggression, and enhanced strength.