(Covers information from Anderson's universe)
The T-virus was a viral organism created by Dr. Charles Ashford in order to reverse the inevitable cellular breakdown in his daughter Angela. The T-virus stimulates pre-and post-mortem cellular growth, however, it kills the host in the process. To counter this, Ashford developed an anti-virus, which would keep the T-virus mutation in check, allowing Angela to live normally and avoid the debilitation that kept her father confined to a wheelchair.
The t-Virus is described by the Red Queen as a major medical breakthrough for the Umbrella corporation, but it also possessed profitable gain for the company if sold to the military. Prior to the events of Resident Evil, the corporation seized the research for the virus from Dr. Ashford and continued to develop it in the underground facility, the Hive. When injected into living tissue, the virus can cause uncontrollable mutation (extreme examples of this mutation are the Licker, the Nemesis and the Axeman).
If infected with the virus, immediate cellar breakdown within the body occurs, resulting in death, subsequent reanimation and an unquenchable hunger for flesh. So far, Alice (and to some extent Albert Wesker), who bonded with the virus and became the equivalent of a superhuman, is the only exception to this rule. The research Umbrella was conducting on the t-virus was considered illegal by environmentalists like Matt and Lisa Addison, who planned to steal the virus and expose Umbrella's work to the world.The nature of the virus is protean; once released into the environment, it adapts accordingly. According to the Red Queen, the virus is both an airborne and blood-transferable pathogen. A simple scratch or bite from the infected persons allows the virus to pass from person to person. In the case of animals, ingestion of infected flesh is all that is required to contract the disease.
During its gestation, the t-virus attacks the adrenal glands, eventually working its way up through the spine to the pineal gland in the brain. The virus may cause violent convulsions in the body just before the pineal gland bursts, killing the victim. The virus then resides inside the brain, making reanimation of the body possible.
The rate of infection varies, depending on the severity of an infected person's wounds. A severely injured Rain lasted less than two and a half hours; Matt experienced immediate mutation from the scratch of the Enhanced Licker. Carlos Olivera went for more than three hours before showing any outward signs of infection. After L.J. became infected, he lasted two days before dying and reanimating.
When the outbreak in Raccoon City occurred and the pandemic decimated the planet, unchecked mutation within the undead populace or further experimentation by Umbrella may have given rise to "Majini Undead", a type of undead with mandibles that sprout from their mouths as a secondary mutation.
In an attempt to replicate the success seen in "Program: Alice", Albert Wesker infected himself with the t-Virus and showed enhanced speed, strength and healing instead of becoming a Undead. The side-effect he experienced for a time was a strong hunger for human flesh. He fed on the Umbrella employees and survivors he captured for experimentation. Wesker has since appeared to have conquered the adverse side-effect. Some time after removing Alice's powers, Wesker injected Alice with the T-Virus, instantly restoring her infection and all of her powers, as she is the "ultimate weapon" in the war against the Red Queen and the Undead horde.
A stronger strain of the t-Virus also exists; created by Sam Isaacs, he treated Undead with a serum derived from the blood of the Alice clones. The resulting "Super Undead" carried a much stronger viral load; once bitten, even multiple doses of the anti-virus could not remove the infection and was implied to have actually exacerbated the situation rather than help. However, all of these Super Undead were killed by Alice and the members of Claire Redfield's convoy and Issacs was killed by Alice and one of her clones. There has been no apparent attempt to recreate this strain of the virus since.
When Umbrella seized Charles Ashford's t-virus, they also took the anti-virus he created to counter its effects. The anti-virus, while able to reverse the degenerative nature of the t-virus within a host, is only effective if the patient does not have a large amount of the t-Virus in their bodies, or the infection is caught in the early stages.