Left 4 Dead pits four human playable or AI controlled survivors of an apocalyptic pandemic against hordes of zombie-like, aggressive "Infected". The Survivors' goal is to help each other make it through various scenarios to safety. The Infected are controlled by an AI that dynamically balances difficulty and mood depending on the players' progress and situation. In an alternate game mode, human players can control up to four mutated Boss Infected with special abilities and cooperate to stop the Survivor players.