(Covers information from Anderson's universe)
The T-virus was a viral weapon engineered by the Umbrella Corporation in the late 20th century. The result of a medical breakthrough in repairing and reviving dead cells, the virus saw profitability as a military weapon. It was ultimately responsible for a catastrophic pandemic which threatened to wipe out humankind.
Accounts differ as to the origins of the T-virus. Two separate scientists have been credited with T-virus research, though both may have took part in research. Both Dr. Charles Ashford and Dr. James Marcus began research on the virus as a means of reversing the degenerative effects of a disease which had afflicted their daughters and, in Ashford's case, had seen the loss of his motor-skills. The virus' ability to repair damaged or dead cells nonetheless made it a medical marvel, and Umbrella, which Dr. Marcus was a senior partner in, was quick to mass-produce it.
The virus itself was fast-tracked across the world, before side-effects could be identified. One boy who received the treatment choked to death on a peanut, which kickstarted the virus' repairing of his cells. He quickly revived in a brain-damaged state, and attacked others. His victims quickly died and, infected with the virus, were revived. Individuals suffering from this were later known as "Undead". While Dr. Marcus wanted the project to be cancelled to prevent further deaths, his counterpart Dr. Isaacs saw opportunity as a military weapon. Dr. Marcus would soon be killed on Dr. Isaacs' orders to ensure it continued development. Dr. Ashford then took over the project and also developed an anti-virus.
By the 21st century, T-virus research had been decentralised, with research conducted in different facilities across the planet. In Siberia, a facility was constructed beneath a Soviet-era submarine pen to study the viability of using mutants created by the virus as weapons for military use, while The Hive beneath Raccoon City performed its own research on the anti-virus. Changing their minds on the military application of the virus, Umbrella's leadership unleashed a pandemic which spread it across the world in an effort to wipe-out the human race by first allowing the Undead to take over, and then release the anti-virus in the air to kill the Undead, offering room for Umbrella's leadership to safely leave their bunker under Raccoon City to resettle the world.
Over the years-long pandemic, Umbrella continued, however, to research means of halting or reversing the spread of the virus. Unaware they were clones of the 'real' leadership, they were not made aware of the anti-virus until a time chosen by the leadership by which it was projected they be released. Among the breakthroughs in research was the Enhanced T-virus, developed by a clone of Isaacs in Nevada. Ultimately, however, the virus was eliminated by way of the intended means by Alice, a clone of Marcus' daughter.
The T-virus typically transmits from host to host through transfer of fluids containing virions, such as infected saliva entering a bite wound. The virus is also known to transmit as an airborne pathogen under certain environmental circumstances.
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. 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 cellular 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 an Undead. The side-effect he experienced for a time, however, 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, claiming at the time that she was 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 a clone of Dr. Alexander 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 Isaacs 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 if the infection is caught in the early stages.