Sir William

Answer: It is never explained how the Nemesis came about in the game. But like the other Bio-weapons, it was probably created in a lab by a scientist. The reason it was created by Umbrella was to kill all STARS members. Because the T-Virus is so highly contagious, if one catches it of course, you turn into a zombie. But, that's not the only thing you can change into. Hence the man in the first Resident Evil movie gets cut, catches the T-Virus and starts to transform into the Nemesis.

Sir William

Answer: Matt from Resident Evil 1 gets scratched by a liquor he then catches the liquor version of the virus. Then as the movie ends you hear the same to say I want him in the Nemesis program which transforms the liquor virus into the Nemesis character. Nemesis is actually made up of several of the bio weapons put together he's another form of the tyrant. Which all together is another character from part 1 he's the boss character from the game.