The genesis of Resident Evil

Albert Wesker's medicine cabinet

The T-Virus (Basic)
The T-Virus was developed through research of the Progenitor Virus, the first mutagenic virus discovered, and combined with the deadly Ebola Virus. The result was a virus that created a mindless, decaying monster, thirsting for flesh - a zombie.

T-Virus (Advanced)
If the T-Virus infection develops, the regenerative qualities of the virus become stronger, with the victim's teeth, nails, skeleton and muscles continuing to develop, creating Chimeras and Hunters

T-Virus (Parasitic)
The whole purpose of the T-Virus was to create super-soldiers called Tyrant (hence the T), but this only happened with certain DNA. It was found that, if attached to a parasitic organism, the results would be consistent.

Developed by William Birkin, this is more volatile than the T-virus. It gives the victim a sense of self-preservation, and so makes them more likely toattack when threatened. The other "advantages" are that mutations increase in size and strength.

T-G Virus
A combination of the two viruses created by Morpheus D. Duvall, an Umbrella researcher who appears in Resident Evil: Dead Aim. The mutations have three stages: the first gives the victim greater size, agility and (seemingly) a sex change, the second an increase in muscle and, lastly, a canine appearance.

T-Veronica Virus
Alexia Ashford worked on the Progenitor virus, combining it with a virus extracted from ants and testing it on her father - turning him into a brainless monstrosity. Through further testing, she discovered that the victim had to be kept in cryogenic sleep, so she injects herself and pops in the freezer for 15 years. On thawing out, she retains human thought and form, but has incredible powers, including flammable blood.

Mystery Virus
Wesker is in possession of another mystery virus, one that has given him his superhuman powers. These were gained by injecting a virus given to him by Birkin, allowing him to fake his death in Resident Evil. It is not known what it is but it could be a more stable version of the TG virus. It might be what RE 4's Krauser has taken.