ε was developed over a number of years due to the Tyrant Project's inability to realise the development of a supersoldier with the existing Beta II strain. This virus was envisioned as one that would increase the likeliness of mutating a human being into a powerful mutant and limiting brain damage. The first confirmed ε mutant was the Tyrant (T-002 Model). An airborne form of the virus was released from the Tyrant's body during a surgical procedure in May 1998, caused in part by an accident directed by the Queen Leech. It spread across much of the facility, infecting a number of staff who, mutating into Zombies, went on to infect more. Unlike earlier t-Virus strains, Zombies infected with "ε" will suffer another set of mutations in response to serious injury, and become "Crimson Heads". These were encountered during the Mansion Incident. A mutant strain of "ε" was later engineered, which instead mutated Zombies into "Lickers".
"ε" and its modified variants are the most commonly-occurring t-Viruses in the series. Most Zombies in Resident Evil; Resident Evil 2; Resident Evil 3: Nemesis; Resident Evil Outbreak and Resident Evil Survivor are infected with it, though not necessarily all.