Left 4 Dead begins in Pennsylvania, following an outbreak of the so-called Green Flu, a highly contagious virus whose symptoms include significantly increased aggression and the loss of many higher brain functions within its victim. The Director keeps things from getting monotonous, however, by throwing in different challenges in every level. The basic gameplay never changes from map to map, so players will learn their way around fairly quickly. Infected players must cooperate to stop them. The goal of Left 4 Dead for the Survivors is to help each other make it through a level to safety. Regular Infected are controlled by the game AI, while human players can control up to four mutated Special Infected with special abilities in Versus mode. The game puts four human playable or AI-controlled Survivors of an apocalyptic pandemic against Hordes of the Infected. During early development, its working title was 'Terror Strike.'

The game was built using Source engine and is available on PC and Xbox 360. Left 4 Dead (abbreviated L4D) is a singleplayer and multiplayer cooperative survival horror FPS game developed by Valve Corporation and Turtle Rock Studios.

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