Umbrella's Changes

Marcus injected Leeches with Progenitor in 1978, and bore witness to the creation of a new strain which he named the "Tyrant virus".^[2] Later this year, the Training facility would be closed and Marcus' two prized students: Albert Wesker and William Birkin, would be moved to the nearby Arklay Research Facility.
Yet again in 1978, did Marcus find evidence of tampering. In his increasing paranoia brought on by his quest for glory and isolation from mankind, he wrote in his diary that Albert and William were the only humans he trusted. His trust in them would betray him ten years later however, as the two would - under direct orders from Spencer, himself - have him assassinated by a Special forces unit and dumped in the Water Treatment Facility. Credit for "T" would be taken by William. His trust in William would also render hiding the source of "T" - the Leeches - useless.
By now however, Birkin had little care for "T", having discovered the G-virus in the body of Lisa Trevor, the daughter of George, still kept in the estate as a guinea pig for their twisted experiments.
Marcus was completely restored by the Queen Leech. Marcus would exact his revenge on Spencer and Umbrella by releasing "T" into the Arklay Research Facility's water supply, turning the facility personnel into zombies and leaving the laboratory and mansion in complete chaos, allowing for the team's extremely dangerous research specimens to escape into the surrounding area.
Birkin and Wesker used this as a diversion for their own plans to leave Umbrella, taking their research with them. Wesker, having formed Raccoon City's elite S.T.A.R.S. unit two years previous, would develop a plan to lure the team to the Mansion and obtain battle data from the conflict that would surely follow. Although the Tyrant was deployed - leading Wesker to falsify his death in a realistic "unforeseen side-effects" of releasing it - both the laboratory and the rampaging Tyrant were destroyed by S.T.A.R.S. Alpha team before making its escape back to Raccoon City with a single Bravo team survivor.
Wesker was forced to go into hiding for a number of months, using his operative Ada Wong to carry out his objectives, including the recovery of Birkin's G-virus. Two months after the Arklay incident, William Birkin finally perfected the viral strain that he'd been toiling to achieve since obtaining Marcus' research so many years earlier. Birkin planned to use the virus to force the Umbrella Corporation into allowing him onto their executive board, but then changed his plans when he found that selling it directly to the U.S. Government would prove far more profitable - though this is based on Umbrella's opinion.
Umbrella deployed its elite U.S.S. Alpha team (a para-military black-operations unit created for Umbrella's most clandestine purposes) led by HUNK in a desperate bid to stop Birkin from handing over his ultimate creation. Although Birkin was mortally wounded in the ensuing conflict, and the G-virus was successfully recovered by HUNK's team, he was able to inject a remaining sample into his own body and devastate most of the team before it could make its escape.
This event would be indirectly responsible for unleashing the T-virus into Raccoon City. HUNK recovered a sample of the virus, extracted and gave it to Umbrella. While the Arklay incident had been relatively isolated and concealed from the public, and Umbrella's control over key government and law enforcement personnel had ensured that any record of what had occurred could be safely swept into obscurity, they were now faced with an entire city infected with "T" and a trail leading directly back to the Umbrella Corporation. This would prove exceedingly difficult, even for Umbrella, to make it simply disappear.