But because I was talking about parasites a while ago (and because I like parasites), here is a bit of news:
Now, there's an idea--an intelligent parasite can only breed in organisms with a certain set of characteristics. Before its mind was subsumed the host left its planet to prevent infecting others. (He was a genius on his world, a twelfth-level intellect on a planet of fifth-level intellects.)
The parasite has caused superpowers because they're a side-effect in humans of the characteristics that the parasite needs.
That might be an interesting end-game to a supers campaign. All sorts of organisms can be infected, but only in supers (or a certain type of super--one of the player characters, perhaps?) can it breed. The PCs have to stop the infection from reaching those individuals and wipe out the infection... Or failing that, wipe out the individual.
Who, I'm sure, is beloved by enough people that it's going to be a battle.