The virus central to Umbrella’s experiments in the original Resident Evil story is the T-virus.
The name is short for Tyrant virus. Umbrella develops it as a biological weapon, and exposure causes severe mutations, reanimation of dead tissue, and the creation of monsters such as zombies and Cerberuses.
The T-virus is associated with the Arklay laboratory beneath Spencer Mansion and with the outbreak that spreads through Raccoon City. Its development also leads to the creation of the Tyrant, a powerful bio-organic weapon.
Resident Evil includes several other viruses, which can cause confusion. The G-virus is connected to William Birkin in Resident Evil 2, while the C-virus features prominently in Resident Evil 6. Uroboros is a later bioweapon from Resident Evil 5.