(Capcom's primary storyline)
P-ε (P-Epsilon) is an Umbrella-created toxic gas. It was designed to stop out-of-control B.O.W.s who have broken out. However, when in contact with Plant 43, it causes them to mutate into a more dangerous form, where they create a poison ability.
Earlier in the franchise development, P-ε was an anti-"t" gas, as it was Lore at the time that only certain enemies were infected with the virus. The Ivies, not infected with "Epsilon", are thus not destroyed by the gas.