Prior to the Great War, the United States government ran an experiment along the Eastern Seaboard by adding to the public water supply a low-dose FEV variant intended to induce radiation resistance. This was done without the public's knowledge, as 1) it was presumed their would be outcry in response to being exposed to something called the Forced Evolutionary Virus, and 2) it was seen as fundamentally benign as fluoridation for the same reason: protecting the general public.
As with most things involving FEV, this Only Kind Of Worked, in the worst possible way. Much like Richard Grey's FEV-II strain (arguably the most consistent FEV variant in the Post-War world) only had a success rate of approximately 8-10%, with the rest simply dying, it's conceivable the FEV strain only worked as -intended in a tiny sliver of the populace. This, coupled with the fact the vast majority of people in the Great War were immediately vaporized by the bombs falling, accounts for the lack of evidence confirming this. However, being a relatively benign FEV strain accounts for the fact that ghouls still occur long after the public water supply has been demolished: it's still a virus, the virus is still contagious, and thus has spread over the last 210 years.
(tl;dr - It's kind of weird to me that Interplay handed Bethesda this 25-kilo ingot of Refined Handwavium and they just kinda... refuse to use it for anything.)
Fallout 2 literally says FEV is airborne and the enclave suspects that everyone outside of them and those in closed vaults have been minorly infected a little bit, if anyone asks about ghouls or psionics or deathclaws or whatever just say it's FEV