Left 4 Dead (L4D) is a new action title from Valve, creators of the Counter-Strike and Half-Life games, that 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. The Survivor Co-op Game Mode is played out across four sprawling campaigns, set in urban and rural environments. Each campaign has an over-riding team objective, and is comprised of five large maps. The campaigns link together to tell the story of the Survivor's escape to safety. The team of four Survivors may be comprised of 1 to 4 human players, allowing for single player and multiplayer co-op games. Next generation AI technology powers all non-player Survivor and zombie characters. This is facilitated by the AI Director, a pool of systems used to procedurally generate a unique experience by monitoring the players' experiences and tailoring the pacing and difficulty of action for each game. This makes for a different experience each time the game is played. In Left 4 Dead's Vs. Mode players may play as a Survivor or as one of four types of "Boss Infected." Each of the "Boss Infected" possesses an exceptional mutant ability - such as a 50-foot tongue lasso (Smoker) or a giant belly full of explosive methane gas (Boome) - instead of traditional firearms.