The Green Flu, commonly referred to as the Infection, is the name given to the unknown virus which caused the current in-universe epidemic in the L4D Universe and maybe possibly other Universes. The virus turns the afflicted into a zombie-like being, making them incredibly homicidal. In rare cases, the infected will not turn and instead become "Carriers." Biologically, the "flu" seems to be a rabies-like pathogen, and CEDA's designation of the virus as a form of influenza appears to be nothing more than a cover-up to avoid mass panic. The origin of the virus is unknown, although it is speculated to be from biochemical weapon experiments. In terms of containment, the Green Flu is likely bio-hazard level 4. Depending of the Infected person or creature, it can turn into a Common Infected, Uncommon Infected or Special Infected. The SI includes the Boomer, Hunter, Smoker, Tank, Charger, Jockey, Spitter, Witch, Screamer, Leaker, etc.