There are parasitic wasps that put their larvae into the caterpillars bodies. In order to create a hospitable environment in the caterpillar's body for their young, wasps also inject virus particles into the caterpillar. The virus particles suppress the caterpillar's immune system so that it can't attack the larvae of the wasps. These virus particles are found in the wasp's ovary, and they can't harm the wasp's organism. The virus particles can't reproduce in the wasp's organism because they are neutered by the wasp.