Several species of parasitic wasps harbour viruses, called polydna viruses,
which they inject into their hosts (insect larvae), together with their
eggs. The virus surpresses the immune system of the parasitoid's host and
allows its eggs to develop (eggs would normally be encapsulated), yet the
virus is harmless to the parasitoid wasp itself. This seems to be a true
symbiotic relationship. Sorry, I have no specific references at hand just now,
this work has been done by Stoltz and others in the 1980s.
Denni (ds4 at st-and.ac.uk)