Smoker Mutations may occur to someone who is/was a heavy smoker. Higher levels of cancer or tar in the victim’s lungs could inhibit complete exhalation and fill the respiratory track with “smoke.” Higher concentrations of the “smoke” in the body may facilitate further mutations. There also could be a reaction to a fungus strain, given the smoky cloud of greenish-brown spores the Smoker gives off. The massive lumps on a Smoker’s body could be full of these spores, which release a cloud of smoke upon death.
Hunter Mutations may occur due to interactions with elevated levels of lactic acid in the muscles and bloodstream—a result of heavy exercise (presumably from a continuous exercise-heavy lifestyle, such as parkour; fleeing from an Infected Horde is rather heavy exercise, but this alone would produce a large number of Hunters).
Jockey Mutations may have resulted from someone who was confined to a wheel chair. The obvious regenerative properties (mutations and growths) of the Infection could indicate a restoration of spinal cord function, giving the infected person the ability to walk again. This would explain the hunchback appearance and the emotional mania. A separate speculation suggests that the Jockey’s mutations may be comparable to the Hunter’s, paired with some form of dementia that either was already present or developed as a result of viral brain damage.
Tank Mutations are possibly related to body building supplements. Abnormally high levels of Creatine, Human Growth Hormone, and anabolic steroids (possibly produced naturally from some damage by the infection) could interact with the virus to promote tumor-like growth of muscle cells. If left uncontrolled, muscle mass would increase exponentially to the point where complex muscle movements are lost or limited and only basic movements (shuffling, climbing, thrashing, slow throwing, etc.) are possible.
Charger Mutations are similar to the Tank and could mean that the Charger is a hybrid version of the Tank. Some steroids are used as an anti-inflammatory remedy for a rash or growth on one side of the body. An abnormal interaction with a localized and smaller concentration of steroids could have created the Charger. It might also be a reaction similar to gigantism (where excessive amounts of growth hormone is released, resulting in thickened bones and asymmetrical body structures), given that the Charger’s facial features and overall body shape is similar to that of someone with gigantism.
Boomer Mutations probably relate to abnormally-high levels of fatty tissue or cholesterol in the bloodstream, which would also explain the large amounts of bile they produce. Bile is created to aid in the breakdown of lipids (fat). The human body’s natural bile production would have been assimilated as part of the Infection’s mutation. This would explain why, when Boomers burst, their torsos appear to be largely hollow. The space previously occupied by fatty tissue had been broken down and converted into a single, enormous sac for containing and producing bile.
Spitter Mutations may have resulted in infecting a human subject with a helicobacter pylori infection (bacterial ulcer). Cells previously infected by the bacterial agent may have been consumed by the virus and caused it to mutate. Another take suggests that the mutations might be similar to the Boomer, except the hormone gastrin (responsible for making stomach acid) is hijacked, resulting in Spitters being able to create large amounts of acid. The Spitter’s sagging skin and awkward walk may be caused by the acid leaking into other bodily cavities and breaking down tissues.
Witch Mutations seem related to an abnormal mental state which could indicate a predisposed mental disorder, like Autism, ADHD, Serotonin imbalance in the brain, etc. Neurotransmitter imbalances are theorized to be a cause of Clinical Depression. Psychological trauma could also be a factor, such as with the “bride Witch”, who may have suffered the mental stress needed to become a Witch when the Infected attacked her wedding. Since the Witch is one of the most notable gender-linked examples of the Special Infected, it is likely that there is also some sort of influence from female sex hormones. It might also be a reaction for anorexia, as all Witches are thin women, and attracted by sugar smell possibly due to lack of carbohydrates.