Left 4 Dead (L4D) is a new action title from Valve, creators of the Counter-Strike and Half-Life games, and promises to redefine the co-operative genre as those titles did for multiplayer and single player action games.

Set in a modern day survival-horror universe, the co-operative gameplay of L4D casts four "Survivors" in an epic struggle against hordes of swarming zombies and terrifying "Boss Infected" mutants.

Valve’s next generation AI technology, allows for games of 1 to 8 players. Players can play as a Survivor or as one of four types of "Boss Infected," each of whom possess a unique mutant ability such as a 50-foot tongue lasso or a giant belly full of explosive methane gas.

The gameplay takes place across four massive campaigns. The "zombie" population of each mission is choreographed by an "AI Director" that monitors the human players’ actions and creates a unique and dramatic experience for them on the fly.

Set in the immediate aftermath of the highly-anticipated zombie apocalypse, Left 4 Dead features single player, multiplayer and co-operative gameplay modes. Next-generation artificial intelligence tailors the pacing and difficulty ensuring a different experience every time the game is played.

Entitled "Crash Course," the latest DLC delivers new single-player, multiplayer and co-operative gameplay to both platforms. Targeted for release in September, "Crash Course" bridges the gap between the end of the "No Mercy" campaign and the beginning of "Death Toll" in the original game, expanding the game universe with new locations, new dialogue from the original cast, and an explosive finale

While containing both Survival maps and a Co-operative Campaign, the primary goal of "Crash" is to deliver a complete Versus mode experience in just 30 minutes, resulting in a streamlined version of the game's existing Versus campaigns. A recharge timer for infected teammates has also been added, and item spawn behavior has changed for more balanced gameplay.

Left 4 Dead: Crash Course is now available free of charge on the PC and for 560 Gamer points on Xbox Live.