The Virus (The Infection) are the infectious, presumably tiny micro-organisms that can infect and turn humans into violent, aggressive zombies that can chase the players when nearby and attempt to kill them. They are the one of the two most frequently found enemy in the game, the second being players themselves. The virus, hence it's name, is probably some sort of viral disease considering how it spreads.
Though not a lot is known about the infection, especially how it really spreads, it is assumed that Trinity Corp. designed (or found) The Virus and began testing on it, as noted by messages found in some buildings. The most well-known, and possibly first infected person, Patient Zero, was experimented on by Trinity to be a weapon but escaped the testing facility and killed/injured many survivors of the apocalypse. The virus then spread and soon the entire map was plagued by zombies. The player must survive as long as possible without becoming zombie food.
The players are immune to the infection, but can still be hurt by the zombies, as shown in the story in the tutorial.