Don't know about humans, but I've recently read that some species of wasp have genes to produce specific type of virus in their ovaries, which, when they inject their offspring into host's body, the virus infect the host to suppress their immune system, so the offspring stays safe. A parasitic system using or supported by another parasitic system. Amazing.

In case you don't know, apparently, some species of wasp inject their offspring into some caterpillars, which then come out of the host's body as larva by piercing through their skin or something. I guess stuff like this is what inspired Alien movies.