A variety of wasp known as the Leptopilina boulardi has very strange reproduction method. The female wasp finds a fly maggot it delays a single egg inside the maggots body. When the wasp egg hatches, the grub will eat the maggot from the inside out. Generally the female wasps are prone to skip a maggot that already has an egg from another female inside, but occasionally two wasp eggs do wind up inside one maggot resulting in wasp grub fight.
However, there is a virus called L.boulardifilamentous virus or LbFV . This virus is able to take over the body of the female wasp as she implants her egg. Once the wasp is infected, she no longer cares if multiple eggs are inside a single maggot. Some maggots will be implanted with as many as 11 eggs according to researchers. Other than co-opting the wasps normal reproductive behavior, no harm is done to the wasp as far as a shorter lifespan.
Interestingly, researchers don't understand how the virus is able to take over the body and change wasp behavior. However, the change in behavior makes it easier for the virus to spread itself since the female wasps transmit the virus to their eggs. The virus is able to move to other eggs nearby as well. If an infected female wasp implants multiple eggs into a maggot implanted with another female wasps eggs, the virus gets another wasp lineage to infect. Researchers are looking into this phenomenon and note that a very closely related wasp called Leptopilina heterotoma isn't bothered by this virus and should one of its eggs find its way inside a maggot with an infected Leptopilina boulardi egg, the heterotoma eggs are generally able to defeat the boulardi grubs that are infected when they hatch without being infected themselves.
[via Discover Magazine]