Left 4 Dead is not a zombie game. These are sprinting and screaming people you're killing here. They've contracted a mutated strain of rabies, but it hasn't made them slow or stupid, it's just made them murderously angry. They don't shuffle toward you in hordes; they run towards you in crowds, snarling with rage. You've never seen anything quite like it. And the noise - imagine the sound of a riot, but a riot where everyone's in agony and hates you. You hear it faintly at first, a distant thunder, but you can tell straight away that it's a lot of people. By the time they come, pouring in from every entrance and banking around corners at blistering full speed, the cacophony is unbearable. This isn't a zombie game, but it is a horror game.
There are four five-level campaigns to shotgun your way through in this disgustingly bloody co-op shooter, and up to three friends can join you in the slaughter - the AI will control any slots not taken by players. Two of the campaigns snake through the city, via hospitals and office blocks, to climactic helicopter rescues on the rooftops. The other two wind around the countryside, where the huge open areas make cutting down the crowds a very different challenge.
Throughout, other players can jump in to take control of "boss" creatures, each of which has a particularly nasty way of incapacitating you, and currently the game doesn't even tell you whether the boss you're facing is a human or AI. But while the maps are pre-made and the bosses often controlled by players, what you actually encounter as you fight through the maps is determined by Left 4 Dead's most important yet invisible feature: The Director.
The Director is an artificial intelligence that controls the number, type and location of the Infected that pour in to swamp you. Its objective is to create an emotional rollercoaster. The perfect survival horror game should hammer you with enemies again and again until you can't take any more, then hammer you a little longer, and finally let up for a while. Close shaves, desperate last stands, health miracles and the out-of-ammo "click!" are always the best moments in gaming, and with the Director, Turtle Rock has found a way to produce them every time.