F.E.V. stands for Forced Evolutionary Virus.
The history of the F.E.V. started with the Americans, trying to 'one-up' the Chinese at a technology race during the Great War that devastated the world. While the Chinese created invisibility type equipment (which the Americans tried to copy poorly with the Stealth Boys), the US came to create two major pieces of technology: The T-51b Power Armour, turning soldiers into walking tanks, and the F.E.V.
Both of these were designed and created by Americas most important private contractor at the time of the war, West Tek.
In 2055, West Tek went underway to create the earliest form of F.E.V. known as the Pan-Immunity Virion Project, developed to combat the New Plague. The New Plague is stated to have killed over two hundred thousand American citizens before the Great War. The New Plague, however, was the first trails of the F.E.V. attempting to find people immune to the New Plague, and so evolve and strengthen. People were in outrage when they found this out, rioting and destroying cities (in order to stop the spread of disease, and partially to protest their treatment).
China became increasingly aggressive with their biochemical projects, and so the US, to keep up, went away with furthering their goal's with the Pan-Immunity Virion Project. In 2075, they officially renamed it the Forced Evolutionary Virus, as their animal test subjects had grown more muscle mass, became more intelligent and increased in size. By the end of 2076, a major amount of progression was achieved and they moved the F.E.V. research to another, newly constructed base, Mariposa. The scientists at Mariposa built giant vats of F.E.V. and began the use on human subjects, who were dipped in to see the effect of the virus on people and learn how to control it. The lead scientists were killed and the base became under military control few days before the Great War had finally errupted and then ended.
The F.E.V. was rediscovered by a scientist called Richard Grey who, after falling into one of the vats on an expedition, gained psionic powers and grew into a tentacled blob of mutated flesh. He then began to call himself The Master and created his own Master Race.
The Super Mutants throughout all of the Fallout series are a direct effect of human contact with the F.E.V. They gained larger muscles, greater size, longer life span and immunity to viruses and radiation. This was to lead to the 'One Race' that would unify the wasteland. However, the more radiated a subject was beforehand, the more resistant their body was to the mutation. Basically, the more radiated you were, the more big dumb and stupid you were to come out. Pure strains of people (namely, those who reside in Vaults) would come out as intelligent and powerful individuals. Perhaps Lou, the Master's Lieutenant, was a vault dweller, as he possess a greater intelligences than any other Mutant in the Fallout games.
A drawback at the time was that the Super Mutants came out sterile and weren't able to breed, setting back the Master's plans (leading him to suicide in one ending of the game). However this was found out to be temporary and the Super Mutants could eventually breed after a long while of waiting (much to their rejoice).
The Enclave eventually found samples of the F.E.V. at the smoldering ruins of Mariposa. They began experimenting with it creating a stronger mix that would kill anyone with levels of radiation in them. Only pure strains of people would mutate. The Enclave were going to use this to wipe out life in the Wasteland in Fallout 2 if the protagonist hadn't of showed up.
Vault 87 on the East Coast of America was fitted with airtight chambers, infecting the members of it with concentrated amounts of the F.E.V. as part of a vault experiment. The Super Mutants there were more yellow, less intelligent, but a lot larger and stronger than the 'normal' Fallout Super Mutants. However, these Super Mutants could not and would never be able to breed and so they had to bring victims back to Vault 87 to create more. They eventually began to run out and had to go searching for the 'Green Stuff'. This was all apart of the Enclave experiment to wipe out life in the Wasteland, as above.
There are five variations of the F.E.V:
- Mariposa F.E.V. Strain
- Vault 87 F.E.V. Strain
- F.E.V. Curling-13 Strain
- Eden's F.E.V. Strain
- Vault-Tec's F.E.V. Strain