Infected (Left 4 Dead)

Infected are the primary threat of the Left 4 Dead video games. Rather than being typical undead zombies, they are living humans that have been contaminated by a rabies-style virus referred to as "Green Flu". This virus has reduced those infected to the state of mindless, homicidal animals that are driven to kill or infect any normal humans they encounter. Their higher brain functions have all but ceased and they cannot communicate or use objects. They attack by using their hands and teeth in frenzied, uncoordinated movements and most likely pass on the virus through fluidic transmission (saliva, blood, etc.).

Common Infected
The most common variety of Infected come in various guises, originating from all walks of life before the Infection. They are attracted by various stimuli, particularly noise and bright lights, and will ferociously attack any uninfected persons on sight. Common Infected are capable of running at an athletic pace and do not seem to tire, likely due to an excess level of adrenaline in their systems. They also tend to attack in large groups referred to as "hordes". They lack any sense of self-preservation and will attack regardless of whatever threats or obstacles are present. In addition, they also appear to be capable of many acrobatic actions, such as climbing fences, climbing walls, and many other actions to get at survivors. While they seem to ignore pain, they are not undead and can be killed just as easily as normal human beings.

Uncommon Infected
Uncommon Infected are somewhat more unique variants of Common Infected, coming from more specific backgrounds and possessing greater abilities and strength. The types of Uncommon are described below.
 * CEDA Worker - Persons working for the Civil Emergency and Defense Agency. These individuals became Infected despite wearing hazmat suits (it is possible they were already contaminated before donning the suits) and the suits make them immune to fire.
 * Fallen Survivor - Individuals who clearly tried to prepare for the zombie apocalypse yet still succumbed to the Infection. They wear militia gear and carry various supplies such as first aid kits, pills, pipe bombs and Molotovs. The armour and helmets they wear make them very tough, but oddly enough they will run away if shot at.
 * Clown - Infected entertainers from an amusement park. The squeaky shoes and honking noses they wear serve to alert other Infected.
 * Mud Men - Infected that dwell in swamplands and use the mud covering their bodies as camouflage. They move on all fours and can run faster in water.
 * Worker - Infected construction workers that wear hard hats and hi-vis vests. Their ear-muffs prevent them from hearing the beeping of pipe bombs and they have greater resistance to blunt melee weapons.
 * Rioteer - Infected riot police. They wear protective helmets and bulletproof vests, making them virtually invulnerable to any frontal attack. They are, however, still vulnerable from behind. When killed, they may also drop nightsticks.

Special Infected
Special Infected are Infected that have undergone horrific mutations, turning them into truly inhuman monsters. These creatures each have a special ability that Common Infected do not share, making them extremely dangerous. The types of Special Infected are listed below.
 * Boomer
 * Hunter
 * Smoker
 * Tank
 * Charger
 * Jockey
 * Spitter
 * Witch