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, the remaining survivors are filled by AI-controlled bots. They play through campaigns fighting off the infected—living humans who have been infected with a rabies-like virus that causes psychosis.[5][6] The overarching goal is not to clear infected, but only to reach the next safe area alive. This is compounded by the presence of an "AI Director", who tracks each player's current situation, and will add or remove items & infected in response; with the goal of maintaining constant tension and creating a new experience with each playthrough.[7]
While a shooter, the game focuses largely on cooperation and teamwork, and thus eschews some "realism" conventions usual in other FPS games. Colored outlines of teammates are visible through walls to help players stick together and coordinate their movement. If a survivor's health is depleted, they become incapacitated and can only be helped by another survivor, after which they continue playing with lower health. If they are incapacitated twice without healing, the third incapacitation will kill them.
During "Campaign" mode, if a survivor is killed, they will eventually respawn in a closet or small room, but must be freed by another survivor to rejoin the team.[8] The AI will not open "respawn closets", so if all human player survivors are incapacitated, the level will restart. Survivors can share first-aid kitsand pain pills, and heal each other. Left 4 Dead has friendly fire, increasing the need for caution & coordination around other survivors.
Via a quick menu, the survivors can communicate and organize with voice commands & callouts.[9] Over 1,000 unique lines were recorded for each survivor.[7] Additional communication of player actions is conveyed through lights; weapon-mounted flashlights and muzzle flashes help players determine when their companions are shooting, performing melee attacks, reloading or moving. (Due to control issues and the likelihood of players using a headset, the Xbox 360 version of Left 4 Dead omits the quick phrases feature.)[10]
