Ok, I'm not sure if I missed something or didn't understand an explanation somewhere, but I've played every game multiple times and I don't remember if it explained how the virus sprreads.

You get bitten by the infected and the virus goes into your cells and controls them I guess.

also dont forget the crows,leeches,and the rats where the first to be infected

It started in the mansion in resident evil one then a crow/raven carried it to Racoon city, people on the outside of the city also got infected by zombies from the mansion.

Actually when Birkin was fightin the Umbrella team in the sewers, he dropped a cannister of T-Virus that somhow infects rats that carry it into the city. The thing is, lots of people are injured but not all turn into zombies, and some that die through other means become zombies.

Watch the FMV near the end of Resident Evil 2

basically there was a biohazard leak at the mansion laboratory but since Alpha Team aka Chris And Jill blew the mansion up the virus disappeared

Then continuing from re2 re3 Birkin gets shot and infects him from the G-virus to keep him alive i guess or get revenge but the spread of the T- Virus was Birkin [ mutated form] killed all the umbrella soldiers and the the tubes which contained broke and the rats got to them [ like the spread of the black plauge in the 1500's i think] rats infected humans.

RECVX how the T- Virus got there was wesker just drop infected zombies [it's my best guess]

and RE4 and RE5 don't contain the T-Virus

there do are some interrogants:

a) how the persons became infected? i dont think that umbrella just went loco and let racoona dn their labs suffer the same desiny.

b) rats migth have been infected but that does not explains a thing.

A. there was a leak in the laboratory [basically the only explaination]

B. Birkin could of spread it or the mice could have spread it:shifty: