The Forced Evolutionary Virus (FEV/F.E.V.) is a major source of mutation in the Fallout universe. Super mutants, the Master, and possibly some ghouls are a result of infection by the FEV. Aside from super mutants, the Master also created a variety of other aberrant creatures using the virus, which includes, but is not limited to: floaters, centaurs, and potentially deathclaws. The FEV exists in various forms across all of the Fallout games, mainly to serve goals of the various antagonists. It was through the FEV that the Master made his super mutant army and how the Enclave meant to purify the world of mutation.