[Name] "Green Flu" (Previously Unknown) [Source] Left 4 Dead, Payday The Heist: No Mercy [Symptoms] 
The Green Flu, commonly referred to simply as The Infection, has caused most humans who come in contact with it to change into homicidal, zombie-like beings. Anyone can catch it, but there are few out there who show no symptoms of the virus. They can, however, spread it to whoever comes in contact with them. These carriers seldom realize they are spreading the virus, leaving a path of destruction in their wake.