The T-Virus is the name of a new type of RNA virus developed by Umbrella. The T stands for Tyrant and was based on pre-existing viral entities. The T-Virus project was intended to create biological weapons that had been outlawed by international treaty and to date the results have been brutal.

The T-Virus takes the genetic code-modifying properties of its Progenitor Viruses and improves on them, resulting in a devilish new form of B.O.W. that fulfills the ambitions of the T-Virus's creator admirably. However, to reach that point, countless experiments were conducted on various living creatures and there have been many failed creations.

The first person to produce notable results from the Progenitor Virus was one of its discoverers, James Marcus. After numerous experiments, he combined the virus with DNA from a leech. The leech immediately underwent transformations and became violent and aggressive. This was the starting point for serious experimentation that made the T-Virus what it is today.

Marcus was so excited by these discoveries that he continued experimenting to prepare the virus for real-world applications, even as he held down his job as managing director of the training facility. In his search for the ideal organism to create controllable B.O.W.s, he tested the virus on arthropods, amphibians and mammals. This is how the monsters known as the Plague Crawler, Lurker and Eliminator were created. Most of them were far from perfect, but the one that showed the most promise was the Eliminator. It had proved that mammals alone had the intelligence needed to understand and obey orders. Marcus threw himself into his research even further and in his quest to uncover the perfect biological organism to use in creating B.O.W.s, eventually he crossed the line and began experimenting on humans.

Afterwards, Marcus's reputation and position at Umbrella fell, the training facility was closed and the T-Virus experiments progressed by leaps and bounds at the Arklay mansion facility. Under William Birkin's direction and after several experiments conducted on humans, they were able to use the T-Virus for genetic manipulation. Not only did the virus cause seeds to grow much more quickly than normal, but by combining it with animal DNA and injecting it into humans, they were able to create monsters known as Hunters. After continued and varied experimentation, the Arklay researchers developed a much higher level of B.O.W. than anything seen before. At long last, they'd created the perfect biological weapon that would obey orders: the Tyrant. With T-Virus experiments at last bearing fruit, the other Umbrella branch facilities began using these new discoveries in their own research to improve on the developments begun at Arklay.

About The Creation Of The T-Virus Vaccine

Were all humans infected with the T-Virus doomed to become unthinking zombies? The answer was no. At the Raccoon City Hospital, a facility that was built thanks to Umbrella's generous donations, doctors were working at a fever pitch to create a vaccine for the "cannibal disease" that had quickly spread throughout the city. The reference material for this was the Medical Devices Manual, written by Douglas Rover, the North American head of Umbrella's medical devices department. After the composite machines' operation was stabilized by sufficient power, a culture fluid base was placed in the machines. The bulk of the manual explained how a combination of this base with the proper vaccine ingredients would create antigen bodies to treat those infected with the virus. But sadly, before the doctors could prepare large quantities of the antigens, they too became zombies and hope for saving the city was lost.

This manual was discovered by a soldier in the Umbrella Biohazard Containment Service, Carlos Oliveira, after the city had fallen into the final stages of destruction. Carlos created the antigen in order to help former S.T.A.R.S. member Jill Valentine, but the majority of the materials explaining the process were lost when Nicholai Ginovaef, who had been sent by Umbrella to observe the rest of the U.B.C.S., blew up the hospital. If one wanted to acquire a copy of this manual, it would have to be from an Umbrella insider. They didn't create large quantities of the vaccine and administer it to the distraught citizens of Raccoon City because they wanted to keep their responsibility for the disaster a secret.