Other new features include new types of special infected and an arsenal of melee weapons. 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. 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. Set during the aftermath of an apocalyptic pandemic, Left 4 Dead 2 focuses on four new Survivors, fighting against hordes of zombies, known as the Infected, who develop severe psychosis and act extremely aggressive. 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. The sequel to Turtle Rock Studios's Left 4 Dead, it was released for Windows and Xbox 360 in November 2009, Mac OS X in October 2010, and Linux in July 2013. Left 4 Dead 2 is a 2009 multiplayer survival horror first-person shooter developed and published by Valve.
