For the often-problematic A.I. issue, Turtle Rock is taking a novel approach, building in routines to allow the game to generate enemies on its own--but to do so in a carefully directed way. "Each and every Infected, whether part of the common horde or a boss, is placed in the world by our A.I. director algorithms," Booth says. "There are no triggers, generators, special flags, or other humanplaced information to determine where these enemies are placed. As a result, the Survivor team cannot predict when or where they will encounter anything."

And enemy placement and numbers won't be the only factors driving the challenge here; individual Infected will be powered by Turtle Rock's celebrated A.I., the tech that drives Counter- Strike's surprisingly intelligent bots.

The result, Booth says, will be a game that's almost infinitely replayable. "A primary design goal," he says, "is to make each campaign playable hundreds of times." And perhaps more important for the game's longevity, it should create an experience that's easily transferred to new maps and scenarios (Turtle Rock is still figuring out whether this 360 game will support user-created content like in the PC edition).

Are we talking about a revolution in the crowded survival-horror genre? Probably not. But with its premise of a handful of humans working together against near-insurmountable odds, Left 4 Dead has the potential to recreate that classic zombie-flick atmosphere with an accuracy no game has yet managed.