Left 4 Dead 2 is a cooperative first-person shooter video game and 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. On September 5, 2010 the game was made available to the Steam client for Mac OS X, and on July 2, 2013 it was made available through Steam for Linux. Valve released the game as a free download for all Steam users for a 24 hour period on December 25, 2013.
Left 4 Dead 2 builds upon cooperatively focused gameplay and Valve's proprietary Source engine, the same game engine used in the original Left 4 Dead. Set during the aftermath of an apocalyptic pandemic, Left 4 Dead 2 focuses on four new Survivors, fighting against hordes of the Infected, who develop severe psychosis and exhibit zombie-like tendencies. The Survivors must fight their way through five campaigns, interspersed with safe houses that act as checkpoints, with the goal of escape at each campaign's finale. The gameplay is procedurally altered by the "AI Director 2.0", which monitors the players' performance and adjusts the scenario to provide a dynamic challenge. Other new features include new types of Special Infected and an arsenal of melee weapons.
The game made its world premiere at E3 2009 with a trailer during the Microsoft press event. Prior to release, it received a combination of positive and negative critical and community reactions. It 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 R15+ rating at the time) and Germany refused to rate the unmodified edition at the time of release. After release however, the game was met with positive reviews.