FEV, or Forced Evolutionary Virus, was a biological agent originally intenteded to counteract the effects of any biological weapons employed against the United States during the Resource Wars. It is the second-leading cause of mutation (next to radiation), and is responsible for the creation of super mutants and the Master.

Before the Resource Wars, there was a heightened paranoia within the top levels of American government that the Chinese would engineer a super-virus that could wipe out entire armies without any expenditure of ammunition or any casualties on the Chinese side. In response, the Pan-Immunity Virion Project (PVP) was formed, and the project was moved to Mariposa Military Base. Eventually a strain DNA-mutating virus was produced that was code-named Forced Evolutionary Virus, and it was tested on various animals and, later, humans. Most results showed increased size, intelligence, and aggressiveness amongst test subjects.

Problems occurred when the military unit guarding Mariposa began to realize the truth. Having been previously kept in the dark, they revolted when they discovered the nature of the program. The colonel in charge of the garrison committed suicide, and his second in command, Roger Maxson, took command and began interrogating the scientists involved. Eventually, disgusted by the vile nature of the research, he ordered them all executed and declared the base to be in secession from the United States. When the bombs dropped, the soldiers inside were protected by the sealed steel bunker. Once things had settled, Maxson led his troops forth into the Wasteland, and with their high-tech equipment eventually formed the Brotherhood of Steel.

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