Not necessarily Phemox.

Over the millenia, there have been multitudes of "naturally occuring" retro-viruses.
These viruses (virii?) can insert their own DNA (or fragments of) into the DNA of its host.
Then when the host re-produces, the child also contains the inserted DNA.
And over time, descendants of the host can become dependant on some or all of that inserted DNA,
and the virus embedded in its DNA may lose parts of itself that it needs to survive on its own.
So they become more of a "merged" entity, rather than host and virus as separate entities.

Thats the way I understand it anyway.

But its probably personal preference whether one would call this a 'mutation' or a 'virus'.
As it is a little bit of both...