Zombies are everywhere in "Left 4 Dead," set in the aftermath of a virtually unexplained undead apocalypse.

Players control one of four survivors of the zombie apocalypse as they try to make it from each level's starting point to a safe house on the other side. It's a typical ragtag group — a college student, a nerd, a grizzled old-timer and a rough-edged outlaw. All of them have the same abilities, though: Run, jump and shoot.

The game doesn't really have a story beyond the setup. It can be played solo, but like the "Battlefield" games and "Team Fortress 2," it's much improved by the addition of a human mind or three.

Oh, and please note: "Left 4 Dead" is violent, gory and gross — just like a zombie movie.

Depending on the stage, the survivors may trek through zombie-infested streets, zombie-infested woods or zombie-infested sewers — wherever they go, it will be infested with zombies.

These aren't the slow, dumb zombies of "Dawn of the Dead" or "Resident Evil." These are the fast, angry, virus-infected zombies of "28 Days Later," and they've brought along some special friends.

The Boomer is a bloated specimen whose vomit attracts hordes of the regular kind; the Hunter pounces, pins and tears at hapless survivors; the Smoker can snag a human with its extremely long tongue and drag him or her away; the Witch will only unleash her devastating attack when disturbed; and the rare Tank is just a mound of mutant muscle.

Each zombie type has a distinctive look and sound; after a few games, it's easy to know the situation just by the noises each zombie makes and they way it moves.

The weapon choices are limited: pistols, shotguns and automatics, along with occasional goodies like pipe bombs.

Downed survivors can be revived by teammates, but only a couple of times. If all four survivors are taken out, it's game over.

The levels are controlled by what the game calls its "AI Director," which alters the playing field, depending on the players' performance, so no level is the same every time. It even decides where to place item pickups.

Players can team up with computer-controlled bots for the game's four campaign settings (each with five areas), play split-screen with friends or hop online.

The game is fun on its own, but working with fellow live brains is better.

And better than that is the versus mode, which sets a team of survivors against an opposing foursome of player-controlled Boomers, Hunters, Smokers and Tanks. (Witches remain computer-controlled.)

This mode offers all the frantic action of the standard multiplayer mode, along with the knowledge that some of those zombies are smart. And playing as the zombies is a blast — they're individually weak but just keep on coming; they can see in the dark and spot survivors through walls; and there's a wicked thrill in snagging a straggler with a Smoker's tongue or pouncing on an unsuspecting human as a Hunter.

After each round, the situation flips, so each team gets to try a level from both sides before moving on to the next.