In the 1960s, Lord Oswell E. Spencer, Doctor James Marcus and his student Brandon Bailey discovered the Progenitor virus in West Africa. The Progenitor virus possessed mutagenic characteristics, which would drastically alter the physical and mental makeup of its host.
Marcus and Spencer planned to use the virus to create biological weapon, forming the Umbrella Corporation along with their mutual friend Sir Edward Ashford. Following Ashford's death in an experiment with the virus, Marcus soon began to work on the prototypes of a malicious biological weapon.
After years of experimentation, Marcus and Spencer realized that the Progenitor virus experienced trouble bonding with its host's DNA. Marcus managed to resolve the issue by infecting a parasitic organism that had a simple-minded purpose in life (to feed, specifically) with the virus and allowing the organism (in this case, a leech) to attach to another organism. This groundbreaking discovery led to more research, eventually leading up to the creation of the t-Virus, named "t" by Marcus. Once an organism is infected with the new virus, it will eventually mutate into a deadly organism.
In the summer of 1998, the staff of the Arklay Laboratory entered phase-3 of their bio-weapons research into the t-Virus. While phase 2 was set about mutating the virus into a more deadly form - Spencer ordered for its potency to be increased from a 90% infection rate to 100%- phase 3 was set at creating a programmable bio-weapon to be used as a soldier rather than a plague carrier like the zombies. Unfortunately for the research staff, humans with the genetic coding capable of cohabiting with the virus were small, just like those able to cohabit with Progenitor. Dr. Birkin's simulation revealed that only 1 in 100,000,000 people were capable of becoming a Tyrant. Dr. Wesker believed that, with more modifications to "t", the number of people compatible with the virus would increase greatly. Unfortunately, for this to happen, a genetically-compatible person was still required to use as a test-subject. Due to the tiny fraction of the human population compatible with the virus, Wesker deduced that ten people would be the highest number Umbrella would be able to find, even if all of North America was searched.
Luckily, Umbrella Europe, who were researching into creating intelligent B.O.W.s rather than strong ones, had success with the Nemesis project. The parasites were designed to control the brain of their hosts and provide the intelligence necessary for the Tyrant program; the two were then planned to be combined once the North American division's "fighting bio-weapon" program was completed. Unfortunately, the parasite was found to kill its host around five minutes after taking control of it. Ordering a parasite from France, Birkin and Wesker attempted to mate the parasite with Lisa Trevor, a woman who had been used by the facility for experiments since 1967. Her absorbing of the parasite into her own genetic coding marked the start of the G-virus project and Birkin's departure from the Arklay Laboratory.