Some viruses have been domesticated by parasitic wasps and use them as biological weapons against caterpillars. However, these same caterpillars steal viral genes and incorporate them into their genomes, protecting them from yet more viruses.

The braconid wasps have a mutualistic relationship with their specific viruses, called bracoviruses. The female wasps deposit eggs and the virus, inside living caterpillars. Bracoviruses suppress the caterpillar's immune system, favoring the growing wasp. The wasps in turn lets bracoviruses multiply in wasps ovaries.

However, sometimes the caterpillar can survive the wasp-virus tandem, either by simply persevering or being lucky, as wasps sometimes attack caterpillars which their virus are useless against. The caterpillars then survive with bracoviruses still in their bodies. These viruses then find their way into the caterpillar's genome.

Sean Schneider and James Thomas from the University of Washington found evidence of bracovirus genes in the genomes of silk moths and the monarch butterflies. They described the wasps as "accidental genetic engineers," modifying caterpillar with viral DNA. These viral genes are now protecting their new hosts from baculoviruses

Read the full article on The Atlantic.