The whole premise of the T-Virus stemmed from work that had been done on the Progenitor Virus (founded December 4 1966) as Ozwell E. Spencer demanded a virus with 100% effectiveness rate from James Marcus, who had asked for huge funding from Spencer. Marcus worked hard trying to create the virus and eventually found the answer by mixing the Progenitor Virus with Leech DNA, thus forming the T-virus in 1978. However, Marcus dreamed of overthrowing Spencer as the Director of Umbrella, a dream he couldn’t live out as Spencer sent his assisstants, William Birkin and Albert Wesker, to assassinate Marcus. After doing so, Birkin became credited as the sole founder of the virus.
As development continued into the virus, Birkin, having created numerous strains of the virus used one, lone test subject, Lisa Trevor. She was subject to extreme biological mutation by the virus but somehow survived.
Birkin continued to use the virus to create B.O.Ws and created the Hunter (mixing reptilian DNA with a human fertilised egg infected with the virus), Chimera (Human and Fly DNA interlinked) and ultimately the Tyrant.
As the virus development reached it’s peak, Birkin used the new NE-α Parasite on Lisa Trevor to see if it would kill her. However, the opposite occured and she absorbed it, creating the G-Virus, Birkin’s crowning moment and ultimate form of the T-Virus.