Left 4 Dead 2

Left 4 Dead 2  is a 2009  first-person shooter game  developed and published by  Valve . The sequel to  Turtle Rock Studios 's  Left 4 Dead  (2008) and the second game in the  Left 4 Dead  series, it was released for  Microsoft Windows  and  Xbox 360  in November 2009,  Mac OS X  in October 2010, and  Linux  in July 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 , the game focuses on four new Survivors, fighting against hordes of zombies known as the Infected, who develop severe psychosis and act extremely aggressive. 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 mixed critical and community reactions, and attracted an unusually high volume of controversy about its graphic content. In response, alterations were made to the cover art, but both Australia and Germany refused to rate the unmodified edition at the time of release. Despite this, the game was met with positive critical reviews, and is considered to be  one of the greatest video games ever made .