T-Virus is the name of the man made virus that turns people into zombies. It was developed by the Umbrella Corporation as one of its secret, biological weapons.
In October 1998, the results of experimentation upon human and animal subjects was discovered by S.T.A.R.S. when investigating reports of murders in the Alklay Mountains. Umbrella Had to bribe the police to make sure no one ever found out what was really going on in Raccoon City.