Left 4 Dead is a first-person shooter, though it makes use of the third-person perspective during cut scenes and some player actions. In Campaign and Single-player mode, the player takes control of one of the Survivors; if four human players are not available, then the remaining Survivors are AI-controlled bots. They play through the levels fighting off the "Infected"—living humans who have been infected with a mutagenic, rabies-like virus to which the Survivors are immune.