Polydnavirus
The virus is injected with the wasp egg into the body cavity of a host caterpillar and infects cells of the caterpillar.  The infection does not lead to replication of new viruses, rather it affects the caterpillar's immune system. Without the virus infection, phagocytic haemocytes (blood cells) will kill the wasp egg. The immune suppression caused by the virus allows the wasp egg to survive. It hatches and develops in the caterpillar.  Also, genes from the polydnavirus in the host alter host development and metabolism to be good for the growth and survival of the parasitoid larva. Thus the virus and wasp have a symbiotic (mutualistic) relationship.