The virus causing the main zombie outbreak in Resident Evil 2 is the Tyrant Virus, commonly called the T-Virus.
Umbrella’s T-Virus contaminates Raccoon City after infected material spreads through the city’s water and sewer systems. Humans exposed to it can become zombies, while animals and other organisms develop different mutations. The outbreak forms the central disaster that Leon S. Kennedy and Claire Redfield must survive.
The game also introduces the G-Virus, a separate biological agent associated with scientist William Birkin. Because both viruses appear in the same story, players often confuse them. The T-Virus is responsible for the widespread undead outbreak, while the G-Virus produces a distinct mutation and drives another part of the plot.
The term “Tyrant Virus” reflects the biological weapons program that also produced Tyrants. Resident Evil 2’s 1998 original and its 2019 remake use the same broad distinction, although the remake changes presentation and several story details.