The Green Flu (Left 4 dead)

The Green Flu is the zombie virus in Left 4 Dead 2 and Left 4 dead.

Offical Facts
(This is from the Left 4 Dead wiki)The Green Flu, commonly referred to as The Infection, is the name given to an unknown virus that has caused most humans who come in contact with it to change into homicidal, zombie-like beings known as the infected. CEDA's designation of the virus as a form of influenza appears to be nothing more than a cover-up to avoid mass panic. Graffiti reading "Not a Flu" can be found throughout left 4 dead 2, meaning that the deception has failed. Promotional materials from multiple sources state that the virus is a mutated strain of the rabies virus. There has been much speculation as to the cause of the infection, or whether or not its origin will even have any relevance in the plot of the game. In terms of containment, The Green Flu is likely bio-hazard level 4.

Trivia

 * It is unknown how the virus fits in with the Venturaintale Role-play, if it is in a different universe, a different time, or doesn't exist at all in the role-play.