Left 4 Dead is a co-operative survival horror first-person shooter video game. It was developed by Turtle Rock Studios, which was purchased by the Valve Corporation part-way into development. The game uses the Source engine, and is available for Windows and the Xbox 360. The game went gold on November 13, 2008, and was released on November 18, 2008, in the United States; and on November 21, 2008, in Europe to coincide with the tenth anniversary of the release of Half-Life. The game pits four people—appropriately called the Survivors—of an apocalyptic pandemic against hordes of the Infected, zombie-like aggressive infected people. There are three game modes: a single player mode in which allied characters are controlled by AI; a four-player, co-op Campaign mode; and an eight-player Versus mode. In all modes, an AI, dubbed "The Director", controls level pacing and item spawns, in an attempt to create a dynamic experience and increase replay value.