Common Infected

Common Infected are the most commonly-faced members of the Infected, and the easiest to defeat. Their strength lies in the fact that they attack in groups, more commonly referred to as the Horde, sometimes as large as several dozen. While they seem capable of ignoring pain, they can still be killed as easily as any normal human can. They are capable of climbing ladders and scaling objects once they find a target, and will not stop until it is dead. The default number of Infected that are spawned in a horde is around 30. These infected are fast, sensitive to loud noises and movement, and able to attack in ravenous waves, as determined by the Director. They might not notice you right away, but when they do, watch out—they're fast, and it won't take long for them to close the distance.

Meet the Horde
Common Infected are drawn to loud noises and bright lights. There are several times throughout the campaigns where the Survivors must do something to alert the horde, such as opening an emergency door that will set off an alarm, and there are other times where the Survivors may accidentally alert them, such as setting off a car alarm or metal detector. All of these trigger something called a Panic Event. When a pipe bomb is thrown, all of the Common Infected in the immediate area will come to investigate the loud beeps and flashing lights, chasing after the pipe bomb and clustering around it until it explodes, killing them. When Common Infected attack from the front, they do approximately 1 damage on easy, 2 on normal, 5 on advanced and 20 on expert. If you are hit in the back, they do 0.5 damage on easy, 1 on normal, 2.5 on advanced, and 10 on expert.

Appearances and Action
Common Infected come in variety of appearances, both male and female. They are the victims of the vicious virus only known as The Infection that is sweeping through the area. The infection is apparently transmitted via bites from other infected.

Common Infected do not differ in appearance from a healthy human as much as Special Infected do. According to the game commentary, the developers designed multiple types of faces and bodies, and would have the computer pair them when spawning Infected, ensuring a variety of characters. This allows for "something around 1500 possibilities". Some Infected models can be seen in any scenario of the game, while others, such as the Infected patients in Mercy Hospital or airport custom officers in Dead Air, are only encountered at certain areas.

When not attacking, they are very vulnerable, usually sitting down, leaning on walls, or simply standing. They are easily provoked by light, sound, or touch, but are normally only dangerous in large groups or when completely surrounding a player. The heads, arms and legs can all be shot off, this happening easiest using a shotgun or hunting rifle. The Infected react naturally to gunfire in their vicinity, turning towards the sound to attack the shooter. Shooting one in the foot will cause a very different reaction than shooting one in the chest with a pump shotgun. Usually, if an idle Common Infected receives a non-fatal gunshot wound from a distance they will writhe in pain and clutch the wound with their hands, but if they are running towards the player and receive a non-fatal gunshot wound they will just continue to pursue the player and ignore the pain. This can be most easily observed by shooting them with a pistol. They are often seen engaging in bizarre activities (e.g. vomiting, attacking inanimate objects, fighting each other) when unaware of the Survivors, but begin to rush at the Survivors the very moment they see them. A Common Infected's attack slows down the target player, making it exceedingly difficult to run away from a mob of them that is close to you. They also seem to be at least smart enough to attempt to trap the player, commonly running ahead of him or her and attacking from in front to prevent movement, assuming there is also a horde behind the player.

The Infected appear to lack a sense of self preservation, as they will actively charge into flames, explosions, off of high buildings, and against superior forces of the Survivors and their weapons, even if they have encountered these things before. A possibility is that the Infection has affected their brains to such a degree that they only think of spreading it, or may simply think of attacking anything that wakes them from their stupor. The Infected also are not known to feed or drink, as in the first level of Dead Air, there is a piece of graffiti that says "They don't eat, they don't sleep, what keeps them going?", and it is possible that the Infection has removed the mental ability to perform these necessary life functions. This may indicate that the Infected may eventually die of starvation or dehydration, and possibly even exposure to the elements as the Infected do not seem to seek shelter in conditions that non-Infected would. However, it's possible that the Infected resort to cannibalism, as evident from the blood on their mouths. It is also possible that the infected just bleed from their mouths. Common Infected's hair and skin pigmentation is usually very low, resulting in pale skin, and sometimes pale hair. Some infected, such as Boomers, Smokers, and other Commons may have black or dark hair. The Infected appear to be capable of many acrobatic actions, such as climbing fences, climbing walls and many other actions to get at survivors.

Extent of Mutations
Though the common Infected have evidently been mutated the least physically by this strain of the infection, the strain appears to have had a bigger effect on their minds. It seems that they have lost many of their higher brain functions, as they seem to be distracted easily by loud noises and flashing lights, and lack a sense of self-preservation, essentially making them mindless cannon fodder to a certain degree. However, their sense of smell and hearing have been greatly enhanced, possibly explaining things such as Crescendo Events and how the Infected are able to notice the Survivors from the littlest things such as the Boomer bile. It's possible they've had very slight boosts in strength, speed, and endurance—though, again, it appears to be slight, as they don't withstand many bullets or hits in general, or because loss of higher brain functions, they seem to ignore pain and damage. They also tend to be incredibly hostile to anything not perceived as being Infected, though there are occasions when two Infected are seen fighting one another, maybe because the infection causes both aggressive and passive behavior. Overall, they seem quite animalistic in nature.

The Common Infected seem to no longer be able to speak coherent language, instead shouting gibberish and screaming at survivors. However, they have been perceived as saying things, though this is probably more a trick of the mind than anything. Some people have heard them saying things such as "let us inside" while in a safe house, or a slightly stranger "I think I might blow a head gasket!" Others have heard them uttering things like "buenos dias", which is Spanish for "good morning". It's possible they are trying to verbally communicate to some degree, though it's slightly irrational as they seem to do more talking with their bodies than anything. It's also possible that, much like the Headcrab Zombies of Half-Life 2 (who often screamed things like "Oh, God, help me!" in reverse), these are actually phrases which have been sped up and/or reversed.

If nothing else, they seem to have some grasp of their current situation, or at least how they feel; generally, idle Infected act physically ill, holding their heads and vomiting as they shuffle around slowly. Occasionally, if you watch one long enough, one of the Infected who is holding their head may begin to writhe in pain, or they may simply lie down and sometimes die without being shot or punched, suggesting that the virus is deadlier than it seems. Or, that after Infection, they have a limited life span, as without treatment, people Infected with rabies (The virus the game's virus was based on) have a limited lifespan. It could also be through brain hemorrhaging, as Infected appear to bleed out the ears, this also explains why they hold their heads in pain. Another possibility is that these deaths may be the result of an injury that the infected person received earlier, such as from a gunshot from other survivors or from a fall that they endured.

Tactics and Tips
Common Infected are only strong in numbers. Sticking together is key to keeping them off you and your teammates' backs. If you are concerned about conserving ammunition, consider using the pistol, which kills them in just a few shots or in one head shot on any difficulty, not to mention it has infinite ammo.

Closet mode
When a horde is about to attack, the best tactic is to back into a corner (preferably) or a wall. Assign the front two Survivors to crouch and melee continuously while the two Survivors behind them stand and fire their weapons at them. This works equally well when Survivors have been covered in Boomer bile. Finding the nearest door, closing it, and using a shotgun to blow out a significant chunk of it provides you with a shield that you can see through, as well as keeping the Infected from touching you—very useful for higher difficulties. The most important thing is limiting the area the Common Infected have to attack from.

Note: This is highly unrecommended for Left 4 Dead 2. Being that the A.I. director will eventually spawn a Spitter that will force you away from your position. Ultimately, this will bring you out into the open, possibly surrounding the player by even more zombies.

Remove loners
Pick off lone Infected that have strayed away from fellow infected whenever you can. When entering a new area, swap to pistols or snipe via a hunting rifle and take out as many lone Infected as you can from the entryway as long as it stays quiet. The Director has a tendency to spawn hordes once you reach the mid-point of areas, and this "event" will cause any idle Infected in the area to attack as well. In Versus, however, it is unlikely the Special Infected will allow you to sit in one place long enough to clear a room in this manner, so keep moving.

Weapon of choice
All weapons possess the ability to fire through multiple Infected, therefore positioning yourself so that the Infected are funneled into lines is effective.

The pump shotgun and the auto shotgun are also very useful against the horde, as their shells deal large amounts of damage and can pass through multiple Infected. The auto shotgun is slightly superior as shotguns go, as it can decimate groups in mere seconds, and there is no pause to pump the gun. A good tactic to use if they are coming through one area, such as a doorway, is to keep your fire pointed at the entry, as the Infected will be clustering together to get through, making them easier to kill. Remember to push the zombies back while reloading simultaneously taking into fact that the shotgun will take much longer to reload than any automatic weapons.

Power of the horde
The Common Infected are especially dangerous on the expert difficulty. It can only take a couple of hits to down a Survivor, depending on the Survivor's current health. On this mode, the Common Infected also deal much more damage: 20 for each hit on the front, and 10 for each hit on the back. This means a horde can easily cripple an entire team if they do not pay attention. The Infected often run up behind Survivors and hit them in the back, downing them before they can realize that they are being attacked. To avoid this, constantly watch behind you and look for Infected sneaking up behind your teammates, but do not shoot when they are close to Survivors to avoid friendly-fire. Instead, Melee them off, then shoot. A good tactic is to always have someone watching behind the group. Communication is also key; always tell other Survivors when they are in danger. Also, it might be a good idea to hop on top of a car or van that has no alarm set because higher ground is usually recommended for fighting large amounts of infected.

Darkness is your friend
The Common infected are obviously known to be attracted by the noise of car alarms and other loud sounds caused by most Crescendo Events, however, most have not realized the flashlight can also enrage Infected. Its effect is similar to that of a pipe bomb, as the bright, flashy lights irritate them, drawing their attention to the player. Using this knowledge may help if two of the Survivors have their flashlights on while the others do not, causing the two "aggro men" to become focused on while their partners are able to break off the rest of the infected.

Playing smart
Common Infected are only a real threat in great numbers, but the Director likes to mess with the Survivors, especially when everybody is low on health and close to a safe room. If this does happen, you can more than likely expect one last horde to come at you; therefore, throwing a pipe bomb can mean the difference from barely limping to safety, and starting all over. The Director will sometimes have Common Infected standing right against the safe room door in higher difficulties.

Infected tips
When playing as the Infected in versus, you can use the Common Infected to your advantage. While the Survivors are attempting to reach the safe room, there are many ways you can exploit the horde. Once the Survivors are occupied by the horde, a competent Tank can defeat even the most organized Survivor team. Boomers can not only summon the horde but also "extend" it by vomiting on the Survivors after they have dealt with the present horde. Smokers are also effective during horde as they can snatch a Survivor and pull them away whilst the others are too busy fighting to help. Smokers can also be protected by the Common Infected. If you have snared a Survivor and you pull him in front of you where you can hit him, the horde will attack along with you, killing them faster. This circle of Infected makes it harder for the Survivors to kill you. You can even do a substantial amount of damage with a Hunter before the Survivors can deal with the horde and kill him. Other Infected will also surround the Hunter to claw at the survivor. There are around 30 Common Infected within a horde. During a horde attack is the best time to work together, Special Infected need to cooperate and doing so with the Common Infected can be fatal to the Survivors.

Uncommon Infected
The Uncommon Common, or the Uncommon Infected, are new forms of the Common Infected introduced in Left 4 Dead 2. 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.

Hazmat-suited Infected (CEDA Workers)
The Hazmat-suited Infected was the first Uncommon Common to be announced and appear in the Left 4 Dead 2 campaign, Dead Center. Players will occasionally come across Common Infected wearing a Hazmat Suit, which was worn prior to the person being infected. When killed, there is a chance of them dropping a Bile Bomb. Upon death, the sound of air hissing out of their now punctured Hazmat Suit can sometimes be heard. Through development of Left 4 Dead 2, the campaign the Hazmat UCI appeared in changed from The Parish, to extend to Dead Center, to being removed from The Parish completely, being replaced by the Riot Cop.

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.

Mud Men
The Mud Men will appear in the campaign Swamp Fever. They are similar to the Common Infected, except that they are covered in mud and use this as camouflage. Mudmen sprint on all fours, although stumble back on two feet when shoved. They also run faster than regular Infected and are not slowed by water.

A Mudman hit obscures the players screen with mud. This increases with successive hits.

Riot Infected
Pre-release of Left 4 Dead 2, Chet stated in an e-mail that "The Parish" would contain new Uncommon Infected, the Riot Infected. While human, they were members of a private security firm hired by CEDA to protect civilians. They became Infected whilst wearing their armor. This gives them immunity to bullets and melee from the front, but not from behind; they are still harmed by fire. Dead Riot Infected may drop a Nightstick or a pistol. Riot Infected were first featured in the demo and were part of the censorship controversy in Australia due to rules regarding violence against authority figures.

Clown Infected
Once a lovable comedian, the Clown Infected is now a victim of the spreading infection. They wander around the Dark Carnival campaign and when they spot Survivors they will charge, calling other Infected in with their noisy, squeaky shoes for an attack. Shove attacks (not melee attacks like with a melee weapon) on a clown's head will cause his nose to honk, awarding the Clownd achievement if this is done ten times.

Worker Infected
When this unfortunate demolition crew receives orders to tear down an old sugar mill, they instead became infected. Dubbed Worker Infected, these Infected populate the Hard Rain campaign. Workers are equipped with ear protection, hardhats, and reflective vests making them easier to spot in the downpour. Their equipment makes them ignore Pipe Bombs or certain Crescendo Events, and their blunt melee resistance is greater than typical common.

Jimmy Gibbs Jr
Jimmy Gibbs is the extremely rare uncommon common infected found in the Dead Center Finale. Being the owner of the car survivors need to escape in, Jimmy is dressed in his racing suit. There will only be one Jimmy Gibbs spawned in the Finale. He has about 1000 health, no detachable limbs and the survivors have unique dialogue when encountering him. A L4D2 game guide hints that he can be found in the Atrium Finale near his car as he has a unnatural biological locater to be drawn to his car.It is also possible that his attack has the same effect of the Mudmen attack (blinding the players screen with mud).

Trivia

 * It should be noted that the Common Infected bear a resemblance to the "zombies" in the movie 28 Days Later, as they are not the undead monsters of traditional zombie culture but living people Infected with a rabies-like virus.
 * Children and animals have yet to be seen in the ranks of the Infected. It is possible that The Infection is too strong for children and animals, so they die instead of turning into Infected. But, since bodies of children are never found, and only several dead cows found throughout Blood Harvest are ever seen, there are a number of possibilities. However, the cows are not confirmed to be Infected, but since they lack the skin around the head, it is possible tests for rabies were conducted on the cattle (As is done in real life). This strengthens the ideas of cows starting or having to do with the start of the Infection, as in Dead Air (inside the airport) there are signs, some with a notice that you are required to wear a mask at all times. Some asking if you have handled livestock (such as cow) in the past few days. Another likely (real world) explanation is that the portrayal of children being injured, mutilated or killed (which they would have to be, by the players, if Infected versions were placed in the game) in video games is forbidden in many places such as Germany: it's easier to simply leave them out than make different versions for the content standards of different regions.
 * Sometimes, a glitch occurs where Infected are standing stationary with their arms parallel at 90º, these Infected can be killed but ragdoll physics do not take effect. Also these glitched Infected can stop Survivors from moving and so it has been seen that Survivors can be trapped by these glitched Infected. To combat this you must restart the server upon which you are playing.
 * A second glitch can occur when one tosses a pipe bomb, such as throwing one towards a horde, and occasionally some Infected may stop moving and glide back and forth, but you are still capable of killing them.
 * Sometimes, Common Infected can get stuck in objects, such as tables or forklifts, and become invulnerable to melee attacks and must be shot to be killed.
 * Found in the sound folder are clips titled to be belonging to a sort of Infected Dog. This could possibly have been a scrapped feature or plans for a future addition. During the 4th chapter of Blood Harvest, it should be noted that howling can be heard in the background, most likely produced from these sound files. However, there is a strong possibility that the file could have simply been applied to the various ambiance that can be heard throughout the campaigns made by idle infected.
 * The voices and sounds of all the Common Infected and Special Infected (with the exception of the Tank and Witch) were performed by vocalist Mike Patton.
 * One of the Infected screams can be identified as one of the GLaDOS AI Component death sounds when you drop one of them into the incinerator in Portal. This is probably Valve reusing sounds in their games, as they reuse several other elements from their past games.
 * This is due to the fact that Mike Patton did the voices for both the Anger Sphere and some of the Infected in Left 4 Dead.
 * It is possible to kill a Common Infected simply by jumping on their heads. To do so, simply walk off an edge which is at least a little taller than the Common Infected, and onto their head. This will instantly kill the poor soul.
 * It is possible that the Infected have grown smarter in the time between Left 4 Dead and Left 4 Dead 2, as in the announcement trailer, Infected can be seen running from an explosion — an act that in Left 4 Dead was considered usually smart. However, it could be that the zombies are just chasing something at the time.
 * When the Infected are summoned in the finales you hear a scream from them. That sound is a mix of two sounds that can be found in the Half-Life sound folder called "the_horror1" and "the_horror2".
 * Common Infected can be instantly killed with a melee in the back, earning the player the "Spinal Tap" achievement.
 * It is worth mentioning that the Infected running towards Coach in the GameStop Pre-Order trailer is a Mud Man, therefore, it should have sprinted on all fours rather than just running, besides the fact that there are no stadiums in Swamp Fever, where the Mud Men make their only appearance.
 * Whenever Common Infected are burned in Left 4 Dead 2 (excluding Hazmat Infected), their bodies gain a new texture effect to make them looked charred.
 * In Left 4 Dead, the Common Infected do not have pupils. In Left 4 Dead 2, they seem to have developed reflective eyes, which glow when you shine the flashlight on them, such as in canines or felines.
 * One of the symptoms right before turning into a Common Infected is possibly getting cold, as the helicopter pilot states he is getting really cold before the events of Crash Course, in which he turns into a Common Infected.
 * There were originally going to be 6 UCI (7 including J. Gibbs), The 6th was called the Fallen Survivor. His ability was when he was killed he would drop a molotov that would immediately ignite. He was cut from the final game for being too overpowered if killed in a horde. He can be seen here with mods. Interestingly enough, his character model is nearly identical to that of Whitaker.