The run-of-the-mill zombies are little more than backup dancers to the five types of “boss infected” that stalk you. Even in large numbers, normal zombies rarely pose a serious threat to well-coordinated survivors, but being forced to deal with the masses creates a perfect diversion that allows the boss infected to close in for the kill. It’s these super-zombies that truly put your survival in the hands of your teammates, because they can pin you down and render you helpless until a teammate comes to your rescue. While the existence of “special” zombies is somewhat antithetical to the entire concept of zombies, the variety and flavor they add to the action is undeniable, and they successfully prevent the inevitable boredom that comes from fighting just one type of enemy. Plus, hearing the disgusting gurgle of a Boomer or the distinctive, haunting wail of a Witch somewhere nearby elevates the tension to a level well above what is safely playable by gamers with heart conditions.

The importance of teamwork may be a point of frustration when playing online, at least at first. The ability to see your teammates’ outlines through walls is a more useful tool for coordination than any screen-cluttering mini-map, but trying to convince everyone to stick close and cover each other is tough even when you’re all experienced gamers sitting in the same room - doing it over the game’s built-in voice chat (or worse, the text-based quick-commands) with anonymous teammates accustomed to playing Rambo-style is a recipe for disaster. Your first few attempts are likely to end in tears, but once everyone has learned their very painful lesson and started to function as a group and listen to each other, the real game can begin.

Survivors are limited to one primary weapon - at the start of each campaign you have a choice of an Uzi or a pump-action shotgun - in addition to your unlimited-ammo backup pistol and a single pipe bomb or Molotov cocktail. In the later parts of the campaign you’ll be able to swap your primary weapon for a more powerful assault rifle, semi-automatic shotgun, or scoped hunting rifle. This is another means by which L4D forces cooperation, because one player cannot carry enough weaponry to handle every situation (and zombies will not let you call time-out to reload).

The current arsenal is a bit anemic, though, and boils down to only three types of weapon: rapid fire, shotgun, and long-range rifle - and the rifle is really only useful in certain wide-open areas where you can use the scope to snipe zombies. But, because the survivors are always on the move you can never hold a sniping position for very long, and the rifle’s low rate-of-fire makes it nearly useless in close-quarters combat indoors. It would have been nice to see some extra weapons (flamethrowers and chain saws come to mind), but there are enough available to get the job done.

No matter how good a shot you are, at least a few zombies will get up close and personal, and there are few things more terrifying than being surrounded by growling zombies tearing away at your flesh. Smacking them with your rifle-butt sends them staggering backwards for a second and gives you some breathing room - time to reload or retreat. But rifle-butting isn’t just a last resort - it’s also a very useful tool that can be used for a variety of tactics. For example, you can use it to push a Boomer back before shooting him, in order to avoid being slimed in the explosion.

Each campaign has a different setting, but all follow roughly the same structure, with five stages separated by safe rooms. It’s not all strictly run-and-gun as fast as you can towards the exit, though - Valve pulls a few tricks to mix up the pace. For example, at certain points you must push a button in order to clear a path - e.g., sending a van crashing through a locked door. Those situations come with a warning of, “Get ready to fight the horde!” This gives you a moment to dig in and think about how best to use your environment to destroy an incoming rush of zombies, such as setting up choke points and throwing down Molotov cocktails in their path, before triggering the rush.