The Infected

The Infected are the "zombies" in Left 4 Dead. They are more akin to the zombies from 28 Days Later than to the more popular slow, shambling type. Instead of being dead, they are living humans who have been infected with a mutated virus. There are six varieties, four of which are playable. Five varieties are considered "Boss Infected" which have special abilities in addition to doing more damage against the Survivors. All AI-controlled Infected are proceduarly spawned by the Director.

Common Infected
Common Infected are the most commonly-faced enemy 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, known as hordes. 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.

Hordes
Hordes are large groups of common Infected that will rush the Survivors randomly and after a Boomer attack.

The Witch


The Witch appears only once or twice per campaign, and is passive unless provoked by flashlights, gunfire, or Survivors getting too close. She is very powerful and is capable of killing players with a single attack as well as sustaining much damage before dying. While the Witch can be killed, it is best to avoid startling her unless absolutely necessary.

The Screamer
The Screamer is a cut unplayable Infected boss that would have appeared once(or more) per map, it would randomly wonder around the map and if it saw the Survivors it would run away and hide, upon hiding it would scream alerting all common Infected to the location of the Survivors, the Survivors would have to hunt it down before it screamed, VALVe thought this was too confusing as they would sometimes have to run through hordes of zombies just to kill something that would summon more zombies, so VALVe removed the Screamer from the game and gave it's "summon common Infected" powers to the Boomer. A sound file and a picture of the Screamer can be found in the Left 4 Dead Folder, by searching for screamer.vtf, and com-thescreamer.wav.

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 he is extremely slow. However, he can vomit on Survivors, attracting the Horde. When he dies, he explodes and covers any Survivors who are close enough in bile. 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, he can pounce players and pin them down until killed or shoved off by another Survivor. Since this "class" is extremely agile, he is also physically weak and cannot take much gunfire.

The Smoker


The Smoker is an ambush class who can strangle Survivors with his long tongue. The help of another survivor is required to escape from a Smoker's hold. As his name implies, Smokers can emit smoke to impair Survivors' vision and voice communication when in danger, or automatically when killed. The Smoker can drag players that he has captured with his tongue.

The Tank


The Tank is the strongest of Boss Infected. Though slow, he has incredible strength and resistance. To counter this overwhelming advantage, the Tank has a 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 of throwing large objects with sufficient force to injure Survivors.

Extra details
When reading some of the ceda posters in the safe house you might see by the step; cover with plastic sheeting and duct tape is a survivors response "not airborne" possibly meaning that the ceda members did not know the exact cause. another such event is in the dead air finale saferoom a flight checklist is on the walls, one step says inform security if you have handled livestock in the last 48 hours.