FEV

The Forced Evolutionary Virus, or FEV, is the mutagen behind the Super Mutants of the Fallout series. In the original Fallout the Vault Dweller learns that humans are being captured and turned into Super Mutants by exposure to the virus, and must travel to the Mariposa Military Base to destroy the vats that hold it; one ending has the player character dunked into the FEV instead. In Fallout 2 the Enclave captures the residents of Vault 13 as test subjects and modifies the virus into an airborne disease meant to kill any creature with mutated DNA. In series lore the Brotherhood of Steel traces its origin to Captain Roger Maxson's mutiny after he discovered the FEV experiments.