The Infected

Left 4 Dead features "zombies" akin to 28 Days Later that come in six varieties, four of which are playable. All Infected are randomly spawned by the Director. The Player controlled zombies have special abilities, making them far more dangerous and intelligent than the Normal Infected.

Normal Infected
Normal Infected are the most common enemy faced and they are the easiest to defeat. They are completely AI-controlled and fairly weak. Their strength lies in the fact they attack in groups, sometimes as large as several dozen. While they seem capable of ignoring pain, they can still be killed as easily as a normal human can. They are capable of climbing ladders and scaling objects.

The Witch
The Witch appears only once per map, and is typically passive unless provoked by flash lights, gun fire, or physically contact. She is easily the strongest infected in the game, and is capable of taking out players with a single attack. She can also take tons of punishment. While she can be killed, it's not recommended to try and kill a witch unless absolutely necessary.

The Boomer
The Boomer is a morbidly obese infected that spits vomit at survivors, which blinds them and draws other infected to them. At a distance, the Boomer is little threat, since they are extremely slow. However, when they die, they explode and cover the survivors in vomit. If a Boomer is killed close to the group of players, the entire group could be doomed.

The Hunter
The Hunter is extremely similar to the fast-zombies from Half-Life 2, although far less charred. The Hunter is extremely quick and agile, and is capable of scaling walls. Additionally, it can pounce players and pin them down until killed. If the hunter remains motionless against a wall, it will turn invisible. Since this "class" is extremely agile, it is also physically weak and cannot take much gunfire.

The Smoker
The Smoker is an ambush class that can strangle survivor with its long tongue. The help of another survivor is required to escape from a Smokers hold. As their name implies, Smokers can emit smoke to distort survivors vision when in danger. The Smoker can drag players that it has captured with it's tongue away from their teammates.

The Tank
The Tank is arguably the strongest of the player-controlled Infected. Though slow, they have incredible strength and resistance. To counter this overwhelming advantage, the Tank has frustration meter that goes up whenever the survivors aren't in view. When the meter is full, the player loses control of the Tank. The tank is capable (with some accuracy) of throwing objects with sufficient force to injure survivors.