
Ok, what about viruses? The human started to transform into an alien because the contact with some strange chemical that triggered a lot of mutations inside his body. The viruses in his body start to mutate also, some of them jump into aliens and start transforming them into humans. Or, at least, in some hybrids between the species. The aliens that left are also infected, and they have to return to avoid infecting their civilisation. But they bring help.