Left 4 Dead is a first-person shooter in which the player takes control of one of four survivors; if 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 rabies-like virus that causes psychosis. Although the survivors are carriers of the disease they do not show signs of any symptoms.
The game focuses on cooperation and teamwork and thus eschews some “realism” conventions usual in most FPS games of the wider genre; colored outlines of teammates are visible through walls to help players stick together and coordinate their movement. If a survivor falls off a ledge, then they may automatically hang onto it and can only be helped up by another survivor. If a survivor’s health is depleted, then they become incapacitated and can only be revived by another survivor, at which point they continue playing with a low amount of health that decreases over time. If a survivor has been incapacitated and revived twice without tending to their wounds, then they will experience distorted black-and-white vision, and the next incapacitation will kill the character. If a survivor takes enough damage while incapacitated, or is not eventually helped up by teammates, then the incapacitated character will die. During “Campaign” mode, if a survivor is killed, then they will respawn in a closet or other enclosed space after a period of time (except during key points in the scenario), but must be freed by another survivor to rejoin the team. Otherwise, the player must wait until the next level. However, if all human player survivors are killed or incapacitated, players will have to restart from the last checkpoint. Survivors can share first-aid kits and pain pills and help each other heal. Left 4 Dead has friendly fire (which causes no damage on the easy difficulty mode), increasing the need for caution around other survivors.