The Infected

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

"Kill all sons 'a bitches. That's my official instructions."

- Ellis

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 the living dead that hungers for human flesh, they are living humans who are thought to have been infected with a mutated rabies virus, that causes increased rage and aggression in the infected and even mutation as in the case of the special infected. There are fifteen varieties of the Infected, seven of which are playable. Eight 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 flashlights, loud noises such as car alarms, or by Boomer bile.

Hordes


A horde is a large group of Common Infected that will rush the Survivors at random intervals, after triggering a car alarm or other Panic Events, Crescendo Event or during a finale.

The Witch
The Witch generally only appears a few times per campaign, and is passive unless provoked by flashlights, gunfire, or Survivors in close proximity. She is very powerful, and is capable of incapacitating/killing 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 cr0wn 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.
 * In Left 4 Dead 2's Hard Rain campaign, a large number of Witches have infested an abandoned sugar mill, which the Survivors must traverse to obtain fuel for their rescue boat. An Xbox Achievement, Sob Story, is earned for navigating the sugar mill without disturbing any of them.
 * The Witch's music cue.

The Wandering Witch
The Wandering Witch is a term used for the Witch in Left 4 Dead 2 during daylight time only, the wandering Witch acts almost the same as the original Witch but rather than just sitting and waiting for the Survivors to come to her, she is able to wander around the map and even surprise one from behind. Apparently she is still zoned out like original Witch, but she cries less.

The Wandering Witch is harder to avoid, leaving the new Survivors in a situation where they must kill her to get past. Apparently, she's able to "sing" instead of cry, when the sun goes down, it is assumed that the wandering Witch will act like the original Witch, sitting and sobbing, waiting 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 Survivors. 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 Screamer didn't have any actual attacks. (However it is believed that it may have been able to use a "headbutt attack" as a final defense.) 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 Survivors, which temporarily 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. In Left 4 Dead 2 there is also a female Boomer, although it is not yet revealed if it will have any effect on gameplay.

The Charger


The Charger is a new Special Infected in Left 4 Dead 2. The Charger runs forward and knocks down Survivors in its way, running after the last Survivor and grabbing them. Once a Survivor is in its grasp, it continues sprinting until it approaches a wall or over hard surface and 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/Smoker. The Charger has much in common with the Tank. Many people theorize that the Charger is actually halfway mutated into a Tank. Even though the Charger isn't as strong as a Tank it can do a large amount of damage. On the more updated version, it appears to have a rocky or calloused surface on it's larger right arm, which may be what it uses to inflict most of it's force when charging.

The Hunter


The Hunter is similar to the fast zombies 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 down Survivors, clawing at their abdomen until either the victim is dead, or the Hunter has been knocked off by another Survivor. 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 Jockey


The Jockey is a new Special Infected that will be appearing in Left 4 Dead 2. It is a smaller, hunched back Infected that attacks Survivors by clinging to them from behind and literally riding the Survivor while attacking them. He can also shift his weight around and cause the Survivor to stumble in his desired direction giving this Special Infected a type of control over the Survivor. It could be a reference to the headcrab in the Half-Life series. The Jockey seems to have a spinal disease, similar to Kyphosis. It also bears a striking resemblance to Gollum from LOTR.

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 Survivor 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 in Left 4 Dead 2. The phlegm bomb she spits contains an acid that spreads upon impact; any Survivor caught within the acid puddle will take small amounts of damage, forcing them to move or become incapcitated. The Spitter's primary role is to separate the Survivors when they hide in enclosed spaces or group up. When Spitter is killed an acid puddle spreads under and around her corpse, causing the same type of damage as her phlegm bomb.

The Tank


The Tank is the strongest of the Special Infected and is reasonably fast, only slowing if hit with automatic weapons fire or walking through flame. If any Survivors at 40% or above health (including health gained from pain pills or the Adrenaline injector) are running away from the Tank, it will not be able to catch them as they can run slightly faster. The Tank has incredible strength and resistance but nevertheless will die within 30 seconds if set on fire, except in Versus. When controlled by a player, the Tank has a "Control Meter" that slowly runs out if the Tank has not hit the Survivors for a period of time. Unless the player is able to locate the Survivors and hit one of them, the control of the tank will slowly lessen, and once it runs out, the player will lose control of the Tank to another randomly assigned player. The Tank is capable of throwing chunks of concrete it rips out of the ground to momentarily stun and damage Survivors and can punch large objects (logs, dumpsters, cars, etc.) with sufficient force to incapacitate the Survivor instantly. In expert mode, the tank can also incapacitate a survivor in one hit, or by throwing a piece of rubble. When idle the Tank makes loud wheezing and rasping noises reminiscent of a rabid animal. The Survivors are alerted of an approaching Tank by dramatic music and furious roars and is dangerously close when the screen starts to shake.
 * The Tank's music cue.

Hazmat-Suited Infected
The Hazmat-suited Infected was the first Uncommon Common to be announced and appear in the Left 4 Dead 2 campaign, The Parish. Valve has also indicated that they may appear in the Dead Center campaign as well. Players will occasionally come across Common Infected wearing a Hazmat Suit, which was likely being worn prior to the person being infected. It has also been said that when they're killed they drop a bile bomb.

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: According to Chet, there is going to be an area that is "literally filled with them". This may be the CEDA strongpoint that was mentioned that The Survivors must breach. A group of them make an appearance in the zombie survival guide trailer walking through flames while the infected around them were engulfed by the fire.
 * Incendiary Ammo (this type of ammo does kill them but not by setting them on fire)
 * Static Fire (e.g. Burning barrels)
 * Gas Canisters
 * Molotov Cocktails

Behind the Scenes

 * All of the Infected's melee attacks are based off of the Counter-Strike: Source Knife programming. This is why the CS:S knife sounds can be found in the game's files.

Trivia

 * The Boomer has been shown to have a female model in Left 4 Dead 2. An interesting thing to note is that Chet Faliszek, the team leader behind Left 4 Dead and its sequel, said that the infected in Left 4 Dead 2 will have variations, looking different from other members of the same infected. Having both a female and male Boomer could be an example of these variations, even meaning there could be female models of Smokers and Hunters, as Chet said some form of variations will effect those two as well.
 * Almost every Infected and deceased being in the game appear to have faded textures in comparison to the colorfully textured Survivors. This is probably to prevent misconception between Survivors and Infected.
 * The Infected seem to have become much stronger in Left 4 Dead 2. In a video released by Gamespot showing Boomer gameplay, the Boomer stumbles much less when melee'd than in the first game. Video:http://www.youtube.com/watch?v=4QKYeVvxb0o
 * The Special Infected that appear in Left 4 Dead have mutated further in Left 4 Dead 2 since the Infection virus has spread further, and possibly advanced. This is shown as the Smoker now has multiple tongues, the Tank has more muscles and less hair, and the Boomer appears gorier.