The Infected are humans that have been mutated by the Redlight virus and Blacklight Virus. During the first outbreak, some were born from Elizabeth Greene's will, such as the hunters, and could be found throughout New York City.
Most were either developed in Hives that were located throughout the city, or bred in infected water towers, which were also prevalent throughout Manhattan Island. During the second outbreak, some forms of the infected were the result of Alex Mercer's will and developed in underground Lairs.
The infected, led by Elizabeth Greene (and later, Alex Mercer), have one aim: to spread the virus across the city, and eventually to the world. The military (and Mercer) stand in the way of their conquest, necessitating the creation of an "Infected army."
During the second outbreak, Mercer assumes a role akin to that of Elizabeth, wishing to create an Infected army and spread the virus throughout the world. Though in Mercer's case, he pursues this goal in a much more active and aggressive way, even resorting to using Blackwatch and Gentek resources and scientists (his enemies) to achieve this.
Infected citizens are ordinary people who have been infected by the Blacklight virus and have undergone minor mutations, turning into walking reanimated corpses virtually devoid of intelligence and driven entirely by their basic instinct to feed and spread the virus by infecting others. Its increased characteristics include increased resistance to damage (or perhaps greater pain tolerance), increased strength, and several different growths and deformities.
Their attacks are lethal to uninfected humans, although they pose almost no threat to armored vehicles and infected beings more powerful. The biggest threat they pose is a minor obstacle, even when they gather in hordes. These infected behave like the generic zombie.
- Main article: Walkers
Walkers are humans who have been infected with a different strain of the Virus than the Infected Citizens.
Most walkers are bare-headed, bloated, and dark-skinned. Others are more lean and lack skin entirely. Walkers are highly mutated, giving them increased muscle mass and numerous deformities, namely the blade on one arm. This blade is not unlike the one wielded by Alex Mercer, though it is vastly inferior.
Carriers can only be distinguished when Alex Mercer has Infected vision active. To put it simply, they are any type of human (Civilian, Military, or Blackwatch) that has been unknowingly infected with the virus, and has not yet begun to show symptoms.
They present no real threat in this state, as they behave in the same manner as their uninfected counterparts. The only instance that they are noted at all is in a mission where Alex consumes multiple Carriers to help achieve a mental link with the Leader Hunter.
- Main article: Hunters
Hunters are the main offensive force of the Infected, and are the most common infected variant that actually pose a real threat to Mercer. They are usually found near Hives and Water Towers, as these are their breeding grounds.
The Hunters are presumed to have been human at one point, since they have a vaguely hominid appearance, however, they have been drastically altered by the virus. The most notable change is their size; they are at least twice as large as the average human being, and much stronger.
- Main article: Hydra
Hydras are large, tentacle-like incarnations of the Blacklight virus. They possess a jagged, horizontal maw at their end that gives them a forked appearance.
A Hydra will emerge from the ground when it senses a threat, such as a tank or a helicopter. When the enemy moves outside of the Hydra's range, however, it will retreat underground once more to tunnel to a more effective location. Heller gains the Tendrils power after consuming a Hydra.
- Main article: Mother
Greene, having near complete mastery over the redlight Virus and its mutations, made the "Mother" form as a defense against the Bloodtox. Erupting from the ground to confront Mercer and the military, she appears as a repulsive mass of infected biological matter standing over 20 feet in height. The mass is supported by something resembling a spinal column, which is protected by three thick, tentacle-like growths.
- Main article: Brawler Hunter
These Infected are similar to Hunters, but they look less human with a more reptilian appearance. Heller consumes one to gain the Claw power.
- Main article: Juggernaut
Large Infected that are more difficult to kill than Brawlers. They try to smash their massive fists at their target, and will try to block melee attacks. The stronger Juggernauts have larger fists and use a ground spike similar to the one used by Mercer. Heller gains the Hammerfist power by consuming one.
- Main article: Goliath
Gigantic Infected with a massive club for a left arm and a spike for the right. To kill one, Heller must remove its right leg, sever the left arm, tear off the head, and consume the body.
- Main article: Flyer
These winged Infected are the only known infected creatures not to be derived from humans (apart from hydras), being the result of infected pigeons. They are significantly larger than their uninfected counterparts and tend to attack in swarms. Out of all the Infected, Flyers are the weakest, and have approximately the same amount of health as Walkers.
- Main article: Evolved
Alex Mercer's spies in Blackwatch and Gentek. They have weaker powers than Prototypes, but they are still very powerful. They are the more intelligent Infected, as they know exactly how to complete their assigned missions. The first Evolved found was Dr. Anton Koenig.
- Main article: Prototypes
Prototype is the fan-term used to describe individuals who have been infected by the DX-1118 C strain of the Blacklight virus. The viral strand genetically rewrites the host's cells, re-purposing it to suit its needs.
- Main article: Super Soldier
Super Soldiers (also known as D-Code Soldiers) are soldiers who have been genetically enhanced by Gentek with the three week old DX-1120 strain of the Blacklight virus.They are only Infected who are on the Blackwatch's side.
- If Thermal Vision ability is used, it can be seen that any infected individual emits more heat than the average human. This means thermal vision can be used if the player is surrounded by smoke to pick out only the infected forces, or to individuate them before they see him/her.
- During the mission before fighting Mother, Hunters and Leader Hunters will burrow to the surface from underground, but none of the Walkers or Infected citizens can be seen doing that. Because only these creatures share the trait only with more advanced virus strains, it might mean that in fact infected creatures are of a subterranean nature, but inferior subjects are used mainly for spreading purposes.
- The Hydra might have been a reference to the Beta Half-Life 2 "Hydra", which is extremely similar as it is a tentacle like the Prototype's "Hydra".
- For some reason, Military/Police forces (including Blackwatch units) doesn't have an infected counterpart, this is probably to decrease development time.