Left 4 Dead 2 is a cooperative first-person shooter video game the sequel to Valve Corporation's Left 4 Dead. The Game 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.