The goal of Left 4 Dead is simple: survive. You are one of the lucky few survivors of a new and highly virulent strain of the rabies virus that has spread throughout the human population, turning the unfortunate masses into rabid, bloodthirsty zombies. You must work together as a team if you want to have any chance of making it out alive. For those of you who don't already know, Left 4 Dead is a first person multiplayer co-op survival horror shooter built on the Source engine. Left 4 Dead pits 4 human Survivors against 4 human or AI controlled boss infected, which with the help of dozens upon dozens of the AI-controlled infected horde, try their hardest to make the Survivors fail.