If FEV is a Virus why cant it spread like one?

[Von Faustien]

i mean so far lore wise you need exposure to an actual source of lab made FEV to mutate but if its a real virus you'd think person to person transmission should be possible right? i mean if you had a cut and super mutant blood got in or say you bit one shouldn't the FEV infection from the mutant tissue propagate into you?

[diegovbl]

That's a good question.
The FEV experiments, shown in the games, implied that the super mutants endured consistent exposure. Mostly airborne (under isolated chambers) and/or dipping into vats containing infected liquid.

If we consider time/consistency would play an important role in "successful" mutations (aka not died), then casual contacts with infected body fluids should not be very effective.
Still doesn't mean the virus is not being spread, though; probably just not enough to change someone's entire phenotype.

[DouglasGrave]

A natural virus inherently has to make alterations that cause the cells it's infecting to replicate the virus itself. If it didn't, it would never spread and infect more than a single cell.

But since FEV is designed and produced independently by humans, it can make virus-type changes to a cell's DNA without needing to propagate itself. In short, an engineered virus doesn't need to grow and spread to exist.