Common Infected

Common Infected, also known as The horde, when they are encountered in large numbers, are the most commonly-faced member of the Infected hordes and the easiest to defeat. They are completely AI-controlled and are 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. The default number of Infected that are spawned in a mob is around 30.

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. 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, 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.

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 degree, but 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. 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
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. Making for 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, while keeping the Infected from touching you -- very useful for expert. The most important thing is limiting the area the Common Infected have to move through, however.

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 (to varying degrees), 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.

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 health. Often on expert mode, the Common Infected end up dealing more damage (they do 20 damage per hit and 10 in the back on expert mode) than the Special Infected as there are so many of them. They love to run up behind Survivors and hit them in the back, downing them before they can realize that they are being attacked from behind. To avoid this, constantly watch behind you and look for zombies 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, tell other Survivors when they are in danger always.

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, throwing a pipe bomb can really mean the difference from barely limping to safety, and starting all the way over. The Director will sometimes have Common Infected standing right against the safe room door on Expert

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 already present horde. Smokers are also effective during horde rushes 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 form a circle around you and the Survivor so they can help you attack. This circle of Infected makes it harder for the Survivors to kill the Smoker. Hunters can even do a substantial amount of damage with a pounce before the Survivors can deal with the horde and kill him. Other Infected will also surround the Hunter to kick and claw at the pounced 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.

Left 4 Dead 2
Common Infected will, of course, reappear in Left 4 Dead 2. They will have more various skins due to the location of the game's setting, Southern United States, with new upgrades in the Source engine, Infected can now survive having limbs, arms, and legs being shot off. They will be bloodier than the original Left 4 Dead and more intelligent. Valve has planned to include uncommon variants of the basic Infected in each of the five campaigns. These Uncommon Infected spawn with the regular Common Infected and presumably cannot be player-controlled. Valve has also hinted that certain areas or Crescendo Events may be almost entirely comprised of the campaign's uncommon infected.
 * Hazmat-suited Infected - These Infected are only seen during The Parish campaign. They are immune to fire and can be seen wearing different colored Hazmat suits, presumably because they worked for CEDA before becoming infected.
 * Mud men - These infected are only seen during the Swamp Fever campaign. They are slightly faster than the normal common infected and scurry around on all fours. If a mudman manages to do damage to a player, that player's screen is partially obscured by a glob of mud, this mud glob grows larger the more damage the mudman does. The mudmen's appearance resembles a normal common infected that is covered from head to toe in mud.
 * Riot Gear Infected - These poor souls faced death even in body armor. The newest Infected have increased protection due to their Riot Gear armor. It is stated (as of now) the only weak spot they have is in the back, however, you may still kill this infected by setting it on fire or melee it. In a recent video though, an infected presumably to be the Riot Suit Uncommon was shot several times in different places, but still did not die.
 * Clown Infected - Dark Carnival introduces a new kind of Common Infected in a clown mask. It can draw small hordes towards the player, similar to the cut infected, The Screamer.

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, perhaps they were killed by the hordes instead. There is also 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.
 * Flies can be heard near corpses and birds may fly overhead, meaning that they may be immune, or just not able to attack the Survivors due to lack of sentience.
 * Once in a great while, a glitch may occur where a Common Infected may become invulnerable to damage, meaning that no matter how many times you shoot or hit it, it will not die. Your AI partners will ignore it, even when it continues to attack them. The only way to "beat" it is to knock it into a room and then close the door, running off while it is still trapped, giving you enough time to escape. Video example of what to do.
 * Another glitch is 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 third 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. This seems to frequently occur around walls and tables—places the Infected think they have to get over or under in order to reach the player.
 * Throughout the game (mainly noticeable in Death Toll), there is barbed wire which the Common Infected can't climb over, as well as barred-up windows which they can't enter through. This is probably a military defense tactic.
 * Found in the sound folder are clips titled to be belonging to a sort of Zombie 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.
 * The voices and sounds of all the Common Infected and Special Infected were performed by vocalist Mike Patton.
 * One of the zombie 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.
 * The Common Infected, if you look at them long enough, will open and close their mouths, even when trapped, and sometimes, when 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, and is known among gamers as killing them "Mario Style", in reference to the way you defeat enemies in the Mario franchise; jumping on their heads.
 * An interesting note is that only male Infected appear to have claw-like nails, females do not posses such a trait.
 * 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 they would have never done. Although, it is possible that the Infected in the trailer were running toward the Survivors, instead, or the infection hasn't completely destroyed they're higher brain functions since the infection has just hit this part of the country, as already announced. This could also be a result of the daytime setting of the trailer and subsequent campaign.
 * 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".