Left 4 Dead is a cooperative first-person shooter set during a zombie outbreak on the East Coast of the United States. Four players control survivors who must fight through hordes of infected humans, creatures transformed by a rabies-like pathogen that causes extreme aggression and loss of higher brain function. Gameplay emphasizes teamwork, with colored outlines showing teammate positions through walls, shared healing items, and friendly fire creating consequences for careless shooting. An AI system called the Director dynamically adjusts enemy placement, item spawns, and pacing based on player performance. Beyond common infected, players face five special mutated types with unique abilities. Campaigns are divided into levels connected by safe houses, each culminating in a desperate last stand for rescue.