The T-Virus was discovered by Dr. James Marcus on September 19, 1978. Naming the virus “t” after “tyrant”, this was the culmination of his life’s work.
The virus itself was based off of the Progenitor, a virus found on a rare flower in Africa. The use of Leech DNA in combination with the Progenitor created a superior virus that could be used for bio organic weapons development.
Once introduced into a human body, the virus kills the host. After a short time, the virus takes control of the brain stem, reactivating the nervous system with primary functions resulting in the most primitive survival instincts. Infected hosts who have reanimated will attempt to eat any living thing they come across, making them a human weapon.
The T-Virus could also be harnessed to create large B.O.W.’s if the gene types were correct in certain hosts. The ultimate goal of the T-Virus was not just to be sold to governments as a means of war fare, but for Umbrella to use the T-Virus to create their own human weapon called the Tyrant. Umbrella was in fact successful in doing so.
This virus is responsible for the outbreak in the Arklay Mountains in Resident Evil Zero and Resident Evil, as well as the apocalyptic outbreak in Raccoon City during the events of Resident Evil 2 and Resident Evil 3.