The Infected

"I'll see peace back on earth if I've gotta murder every one of these bastards with my own bare goddamn hands!"

- Bill

"Call these freaks whatever you want!"

- Francis

"If it's not one of us... kill it!"

- Zoey

"Humanity's gonna bounce right back, you watch!"

- Louis

The Infected are the "zombies" in Left 4 Dead. They are more akin to the antagonists from 28 Days Later than to the more popular slow, shambling type. Instead of being dead, they are living humans who are thought to have been infected with a mutated rabies virus, though this has not been confirmed. There are six varieties of the Infected, four of which are playable. Five varieties are considered "Boss Infected", or "Special Infected", which have special abilities in addition to doing more damage against the Survivors. All AI-controlled Infected are procedurally spawned by the Director.

Common Infected
Common Infected are the most commonly-faced enemy, as well as the easiest to defeat. They are exclusively AI-controlled and fairly weak. Their strength lies in their tendency to attack in large groups of up to several dozen, known as Hordes. While they seem capable of ignoring pain, they can still be killed easily. They are capable of climbing ladders and scaling objects, as well as breaking down weak walls. This type of Infected are attracted by loud noises or bright lights.

Uncommon Infected
In Left 4 Dead 2, new forms of the Common Infected will be introduced, called the Uncommon Infected. They have the same general weaknesses and health as most Common Infected, but there are a few differences that can affect how they're dealt with. Valve has hinted that there will be a different variety of Uncommon Infected in each of the campaigns. So far two have been revealed:

Hazmat-suited Infected
Occasionally, in the Left 4 Dead 2 Parish campaign exclusively, a player will come across Common Infected wearing a Hazmat Suit, which was likely being worn prior to the person being infected. Two can be seen in the beta game-play in part 3, around 3 minutes and 46 seconds in.

Though not as numerous as normal Common Infected, they share the same health, abilities and weaknesses. However, due to their Hazmat Suits, they are immune to fire in all forms. This includes:
 * Incendiary Ammo.
 * Static Fire (e.g. Burning barrels).
 * Gas Canisters.
 * Molotov Cocktails.

Mud Men
In the campaign Swamp Fever, the Survivors will meet another type of the Uncommon Infected, known as Mud Men. They are similar to the Common Infected in almost every way, except that they sprint on all fours, exceeding the speed Common Infected reach when running. This makes it particularly harder to kill, especially when traversing through water. When a Mud Man hits you, your screen is partially covered in mud, the more it hits you, more mud covers your screen.

Hordes
Hordes are large groups of Common Infected that will rush the Survivors at random intervals, after triggering a car alarm or other Panic Event, Crescendo Event, during a Finale, and during the aftermath of a successful attack.

The Witch
The Witch appears only once or twice per campaign, and is passive unless provoked by flashlights, gunfire, or Survivors in close proximity. She is very powerful, and is capable of incapacitating players with a single attack as well as sustaining much damage before dying. Luckily, many hints are given to let Survivors know when a Witch is nearby. The Witch cries loudly, which Survivors will mention they hear, and dramatic music will play. While the Witch can be killed, it is best to avoid startling her unless absolutely necessary and remember to turn your flashlight off. The Witch is very rarely found in small rooms and normally stays off the main route but has been found in front of safe rooms, doorways and in the middle of streets. Once a Witch has killed a Survivor she will retreat out the area and completely disappear. If a Witch must be killed then you have one of two options. 1. sneak up on the Witch and attempt to crown it with the Auto Shotgun. (If the initial head shot connects but does not kill only one or two more shots should kill it.) 2. Have one player move far back, as far away from the Witch as possible. The Witch will target the player that disturbed it, meaning it will attack the farther away player providing he shot or shined a flash light on it. This will allow the other players to shoot the Witch as it is running to the farther away player. When using this method watch for Special Infected as they may ruin the plan or even kill the team if they attack at the right moment.
 * The Witch's music cue.

The Wandering Witch
The Wandering Witch is a new infected introduced in Left 4 Dead 2 in Daylight time only, the Wandering Witch acts almost the same as the original Witch but rather then just sitting crying for the Survivors to come to her, she is able to wander around the map and even surprise one from the back. Apparently she is still zoned out like original Witch, but she is crying less. The Wandering Witch is harder to be avoided, leaving the new Survivors in a situation that they must kill her to get past her, apparently she's known to be able to "sing" instead of crying, when the sun goes down, it is assumed that the Wandering Witch will act like the original witch, sitting and sobbing wait for Survivors.

The Screamer
The Screamer is a cut AI-Controlled Special Infected that would have appeared one or more times per map. It would wander around until it came in contact with the. Once it saw them, it would quickly run away and hide. When it was successfully hidden, it would let out a scream that would attract the Horde.

During testing, play testers complained that the Screamer would end up being too hard to spot in a crowd of Infected. Its horde-attracting scream ended up evolving into the Boomer's bile. The reason is because the Screamer didn't have any actual attacks. Its only ability was to summon the horde with a scream. When the Screamer was removed, The Boomer inherited its horde summoning abilities in the form of the vomit attack, and the bile death explosion.

A sound file describing the screamer 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 an Infected that vomits a special bile at, which blinds them and draws Common Infected to them. At a distance the Boomer is little threat, since he is extremely slow and weak. However, if within range he can vomit on Survivors, attracting the Horde. When he dies, he explodes and covers any Survivors who are close enough in his bile. If a Boomer is killed, nearby Survivors will additionally be blinded, and will stumble backwards. Boomers can be anticipated because of the gurgling noise they make.

The Charger
The Charger is a new Special Infected in . The Charger runs forward and knocks down Survivors in its way, running after the last Survivor and grabs them. Once in its grasp, it starts slamming them into the ground until an ally frees them. Despite the fact that the Charger is almost the size of the Tank, it does not have as much health, but it still has more than the Hunter.

The Hunter
The Hunter is similar to the from Half-Life 2, although they are not as mutated on the outside. The Hunter is extremely quick and agile, capable of quickly scaling walls. Additionally, he can pounce and pin(incapacitate), clawing at their abdomen until either the victim is dead, or the Hunter has been knocked off by another Survivor or killed—they are not the most physically adept Infected, having relatively low health, and thus it doesn't take very much to kill them. When the Hunter crouches down in preparation to pounce, it growls, giving it away to any nearby Survivors. However, whilst standing it makes no noise. The Hunter is the only special infected that can remain absolutely silent.

The Smoker
The Smoker is an ambush class who can strangle Survivors with his long tongue, similar to Half-Life ' s Barnacle. The help of another is required to escape from a Smoker's hold. As the name implies, Smokers emit smoke to impair Survivors' vision and voice communication when killed. A Smoker also will frequently cough, letting the Survivors know when he is in the area. The Smoker can drag players that he has captured with his tongue towards himself, though he remains stationary and cannot move. The Smoker is also momentarily paralyzed for a few seconds after having his tongue removed from his captive by another Survivor.

The Spitter
The Spitter is a new Special Infected introduced Left 4 Dead 2. It's primary task is to spit at the Survivors. The phlegm bomb it spits contains an acid that spreads upon inpact. Any Survivor caught within the acid puddle will take small amounts of damage, thus prompting them to move. It's an Infected that has to support the Infected team much like the Boomer, while both support their team, the Spitter's goal is to successfully separate the Survivors and cause them to stop using and even run off.

The Tank
The Tank is the strongest of the Special Infected, though also one of the slowest. If any Survivors at full health are running away from the Tank, the Tank will not be able to catch them as they both run the same speed. Even though the Tank has incredible strength and resistance, it is also highly flammable, and will die within 30 seconds when set on fire. When controlled by a player, the Tank has a "Control Meter" that slowly runs out whenever the Survivors aren't in view. Unless the player is able to locate the Survivors and hit them, the control of the tank will slowly lessen, and once it runs out, the player will lose control of the Tank. The Tank is capable of throwing large objects (such as blocks of concrete, cars, or tree trunks) with sufficient force to momentarily stun Survivors, or even incapacitate them instantaneously. The Survivors are alerted to the approach of a Tank by dramatic music, and the loud pounding and furious roaring a Tank makes when it runs. You will know that a Tank is dangerously close by when the screen starts to shake, ominous music plays, and when Survivors start screaming.
 * The Tank's music cue.