Left 4 Dead 2 (abbreviated as L4D2) 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 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, who develop severe psychosis and act zombie-like. 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. After release however, the game was met with critical acclaim and positive reviews.