if you play through Resident Evil (Remake GC) and Resident Evil Zero, both explain how the T/V-Virii work.
Basics of it is, what they initially created and is released you must die soon after infection. Otherwise the bodies immune system kicks in an kills the virus as they are very weak variations used for testing purposes. When they move forward with the G-Virus, which is based on the Las Plagras to create the mutating "Nemesis" symbiot that is when the virus is then capable of surviving against the hosts immune system.
T/V-Virii are based on the common cold, the G-Virus is a retro/symbiot.
There's craploads of the documentation in both GC-only titles that delves extremely deeply into the history of the virus and the outbreaks.