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 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.
The game is focused on cooperation and team play; 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 incurs enough damage while incapacitated, or is not eventually helped up by teammates, then the incapacitated character will die. 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. Survivors can share first-aid kits and pain pills and help each other heal. Left 4 Dead has friendly fire that cannot be disabled, increasing the need for caution around other Survivors. On the Easy difficulty level, friendly fire does not harm teammates but still registers as having occurred with a notice on the screen saying, "Don't shoot teammates!"
The Survivors communicate by voice commands that are accessed by quick menus, and some sound off automatically when performing actions such as reloading or spotting Infected. Over 1000 unique lines have been recorded for each Survivor. Additional communication of player actions is conveyed through character lights. Also, weapon-mounted flashlights and muzzle flashes help the players in determining whether their companions are shooting, performing melee attacks, reloading or moving. Due to control issues and the likelihood of players using a LIVE headset, the Xbox 360 version of Left 4 Dead omits the quick phrases feature.
The game is experienced through four campaigns that take place in various urban and rural locales. Multiple visual in-game hints—including license plates, park signs, and markings on airport equipment—imply that these locations are in Pennsylvania. Each campaign is divided into five chapters marked by safe rooms, which are checkpoints where players can heal, re-arm and revive characters who were killed. Specifically, the four campaigns are: "No Mercy", an urban setting culminating in a hospital; "Death Toll", a small-town and countryside setting; "Dead Air", an airport setting; and "Blood Harvest", a woodland and farm setting. The levels are essentially linear, with distinct beginnings and ends, but there are a number of alternate routes to follow with more supplies, helping to create a sense of non-linearity. In the final chapter of each campaign, the players must defend a position from an onslaught of Infected until rescue arrives. Each campaign typically lasts between 35 and 75 minutes depending on the difficulty level. The Windows and Xbox 360 versions of the game both contain achievements.