Left 4 Dead 2 is a cooperative first-person shooter video game. It is the sequel to Valve Corporation's award-winning Left 4 Dead. The game launched on November 17, 2009, for Windows and Xbox 360 in the USA and November 20 in Europe.
Like the original, L4D2 is set during the aftermath of an apocalyptic pandemic, and focuses on four survivors fighting against hordes of the infected, who develop severe psychosis and exhibit zombie-like tendencies. The survivors must fight their way through levels, interspersed with safe houses that act as checkpoints, with the goal of reaching a rescue vehicle at the campaign's finale. The gameplay is procedurally altered by an artificial intelligence engine dubbed the "Director" that monitors the players' performance and adjusts the scenario to provide a dynamic challenge. Several new features have been introduced: new types of infected, melee weapons, and a story-arc that connects the game's five campaigns together.