Common Infected

Common Infected, also known as The Horde, are the most commonly-faced enemy and 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. The default amount of infected that are spawned in a mob is around 30.

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.



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. 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. 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.

The infected appear to lack a sense of self preservation, as they will actively charge into flames, explosions, 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, and that this is possibly what the infection is attempting to do, which is spread as far as possible. The infected also are not known to feed or drink, and it is possible that the infection has caused them to no longer 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. according to the game commentary, they designed multiple types of faces and bodies, and would have the computer pair them when spawning infected, ensuring a variety of characters. this allowed for "something around 1500 possibilities".

Extent of Mutations
The Common Infected have been mutated by this strain of the infection, though not by a large degree. They have apparently lost many of their higher brain functions and a sense of pain or self-preservation, and possibly their sense of smell and hearing have greatly improved. They tend to act viciously against anything not perceived as being infected. Their mentality has greatly decreased, and they are easily distracted by loud noises and flashing lights. They do however seem to have some minor understanding of their situation, some idle animations show them struggling with migraines, holding their head, etc.

Tactics
Common Infected are only strong in numbers. Sticking together is key to keeping them off of 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.

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.

Pick off lone 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 boss infected will allow you to sit in one place long enough to clear a room in this manner, so keep moving.

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.

The common infected are especially dangerous on the expert difficulty, it only takes a couple of hits(depending on the amount of health the survivor has) to down a survivor. Often on expert the common infected end up dealing more damage than the special infected as there are so many of them. They love to run up behind survivors and claw 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.

Trivia

 * No children or animals have yet been seen among the infected.
 * 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 stood 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, and become invulnerable to melee attacks and must be shot to be killed.
 * 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.
 * Interestingly enough, they sometimes make noises that sound like garbled, rasping speech.