The T-Virus, or 'Tyrant' Virus, was unveiled as the virus that was causing all the fuss in Resident Evil one. Being researched in the Labs beneath the Mansion in Resident Evil 1, the T-Virus was initially intended as some sort of super drug for the army. Wesker wanted the drug to be a sort of chemical weapon to turn all soldiers into biologically enhanced killing machines.
Naturally, being the hell bent maniacle psychopath, Wekser's research goes astray and starts to mutate things. People, plants, insects and. Some to a small extent, other to a large one, breeding creatures like The Tyrant or Plant 42.
The Virus is transmitted from host to host by direct contact of bodily fluids. The day the virus goes airborne, the world ends. For the time being, it's transmitted by a bite, a cut from someone/thing who is infected or the saliva of one to another.
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