From the 2nd movie of the series, Resident Evil, the Nemesis. After the apocalypse happens, Umbrella Corp. dropping the vial of the T-Virus, majority of the city is infected. Later on scientist take on a guy at the end of the first movie to do research on a "Walking Weapon" for say using the T-Virus in side him, therefore the appearance of the Nemesis, as a result of the research, in the 2nd movie.