Left 4 Dead: Out for Blood/Zombies

The Infected (sometimes referred to simply as Zombies by the survivors) are Left 4 Dead's take on the classic movie and pop-fiction zombie. Rather than being undead, they are living humans who have been Infected by the Green Flu virus, causing massively increased aggression and loss of many higher brain functions such as speech and self-preservation. Like cinematic zombies, those present in the Left 4 Dead series attack human victims until they themselves perish or kill their target.

Characteristics
In the cases of Common and Uncommon Infected, observed aggressiveness manifests itself primarily in the form of frenzied physical blows and kicks. Bite attacks are of a secondary nature and may even be incidentally delivered. The Infected primarily attack uninfected humans who have attracted attention through noise, movement, displaying bright lights, or anything covered in Boomer Bile (whether it be from a Bile Bomb or the Boomer itself). Common Infected are also frequently observed displaying mutual aggressiveness towards one another. However, these attacks cease when the Infected find and focus on an uninfected human.

It seems that psychologically, both Common Infected and Special Infected cannot remember their lives before they turned. For example, Carolyn attacked her husband Wade shortly after contracting the Green Flu, Sandra tried to kill Francis mere moments after talking with him normally, Church Guy tried to kill the survivors right after opening a door for them, and Jimmy Gibbs Jr. will attack Coach and Ellis even though they will recognize him and call him by his former name.

The Infection might also have changed their physiology. Some Infected suffer severe tissue damage and dismemberment from moderate blunt force, yet they can still jump down from heights of approximately 20 feet with perfect physical coordination and without harm. They have no fine motor control and their gross motor control functions are heavily degraded, thus rendering them incapable of picking up objects and operating simple mechanisms such as door handles.

When passive and unstimulated, Common Infected lean against a support (such as a wall), sit, lie recumbent or stagger aimlessly in short uncoordinated steps. Many will vomit profusely or display visual signs of being unwell (e.g. slumped over cradling the head). The Common Infected have also been observed attacking and killing other Common Infected. The cause of this is unknown. Special Infected and Uncommon Infected tend to be less inert in their passive state since they are invariably observed to be in a standing position.

When active and stimulated, Common Infected display extremely contrasting characteristics, as their speed of movement is rapid, their gait is highly coordinated and their focal direction unerring. Uncommon Infected display the same behavioral qualities with the exception that they appear to be more sensitive to external stimuli and are thus more easily aroused and provoked, with some having abilities to draw other Infected.

Through some unknown mechanism, a small percentage of Infected humans will undergo a dramatic mutation, eventually turning into one of the Special Infected. There are eight varieties of these Infected, seven of which are playable. These eight varieties have special abilities in addition to doing more damage against the Survivors. In turn, two of these eight varieties are considered "Boss Infected", namely the Tank and the Witch.

Common Infected
Common Infected, as their name implies, are the most commonly faced enemy and are the easiest to defeat. They are more akin to the antagonists from the film 28 Days Later than to the typical shambling cinematic zombie. 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.

However, the danger they pose must never be underestimated. Common Infected can easily bring down health on Expert as every hit drains health by 20 (10 if hit from behind), unlike Easy (1 from front, 0.5 from behind), Normal (2 and 1) and Advanced (5 and 2.5). 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 and doors. They cannot break down safe room doors, so you should be relatively safe inside of a closed safe room. This type of Infected are attracted by flashlights, loud noises such as car alarms, or by Boomer bile. They appear to lack self-preservation, but do appear to have some intelligence, such as in the Hard Rain or The Passing campaign: the Horde will attack when the storm picks up and there is minimal visibility.

Throughout Left 4 Dead, the physical appearance and dress of Common Infected are consistent with the particular location being visited. For example, in Mercy Hospital, the Infected are medical staff, patients and maintenance personnel, whilst at Metro International Airport, Common Infected include flight, cabin and ramp crew plus business executives. In more neutral locations such as Riverside or Daughtery Farm, Common Infected appear as generic "everypersons, " including servicemen wearing camouflage, blue collar and office workers and young adults through to late-middle-aged individuals of both sexes. The same principle applies throughout Left 4 Dead 2, but given this game's diverse locales, the range of physical variation displayed is considerably wider relative to Left 4 Dead.

Horde
A Horde is a large group of Common Infected (sometimes Uncommon Infected included and rarely Special Infected) that will rush the Survivors either at random intervals, during the finales, as a response to Boomer bile/bile bombs, or in response to loud, high-pitched noises and/or bright lights (car alarms, most other alarms or noises during Panic Events, pipe bombs). Common Infected are weak when alone, but are strong in number. Be alert and prepared when you hear the music cue of an incoming Horde. The Horde can easily bring down any uncoordinated team, especially on Expert.

Uncommon Infected
Left 4 Dead 2 & 3 introduced special types of Common Infected known as "Uncommon Infected". Each Uncommon Infected is unique to the campaign it appears in (excluding DLC campaigns) and has its own special ability that makes it slightly to moderately more dangerous than Common Infected.

Firefighter Zombie
"'Those guys used to be NYFD's finest. Now? They're just brainless zeds wielding damn fire axes.' - Nick."The Firefighter Zombie was introduced in the "Central Park" map in the "New York City" region in L4D3. It wears an NYFD uniform, and is sometimes seen wielding an fire axe.

Their limbs are detachable but players cannot cut them in half, even with a chainsaw. This fact means the hazmat suit was intact after its occupant had become Infected. The most logical explanation is that hazmat suit wearers were infected prior to donning their suits, thus rendering this ostensibly protective gear redundant.

Lion Zombie
"'I hear the Infection goes farther than just humans. We've seen zombified dogs tear through survivors in a matter of seconds' - Nick"