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