Left 4 Dead 2 is a cooperative first-person shooter game. It is the sequel to Valve Corporation's award-winning Left 4 Dead. The game launched on November 17, 2009, for Microsoft Windows and Xbox 360 in the United States and November 20 in Europe; in 2010, Left 4 Dead 2 was made available to the Steam client for Mac OS X. It builds upon the cooperatively-focused gameplay of the original and uses Valve's proprietary Source engine, the same game engine used in Left 4 Dead. The game made its world premiere at E3 2009 with a trailer during the Microsoft press event.
Like the original, Left 4 Dead 2 is set during the aftermath of an apocalyptic pandemic, and focuses on four survivors fighting against hordes of the infected. 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 adjust 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.
When it was announced, it received a combination of positive and negative critical and community reactions. It also attracted an unusually high volume of pre-release controversy about the game's graphic content. In response, alterations were made to the cover art and both Australia (due to having no R18+ rating) and Germany refused to rate the unmodified edition.