Ed Yong in The Atlantic:
This is a story about viruses that became domesticated by parasitic wasps, which use them as biological weapons for corrupting the bodies of caterpillars, which in turn can steal the viral genes and incorporate them into their own genomes, where they protect the caterpillars from yet more viruses. Evolution, you have outdone yourself with this one.
The wasps in question are called braconids. There are more than 17,000 known species, and they're all parasites. The females lay their eggs in the bodies of still-living caterpillars, which their grubs then devour alive.
As early as 1967, scientists realised that the wasps were also injecting the caterpillars with some kind of small particle, alongside their eggs. It took almost a decade to realise that those particles were viruses, which have since become known as bracoviruses. Each species of braconid wasp has its own specific bracovirus, but they all do the same thing: They suppress the caterpillar’s immune system and tweak its metabolism to favour the growing wasp. Without these viral allies, the wasp grubs would be killed by their host bodies.
So, the viruses are essential for the wasps—but the reverse is also true. Unlike most other kinds of virus, these bracoviruses cannot make copies of themselves. They are only manufactured in the ovaries of the wasps, and once they get into the caterpillars, their life cycle ends. Some might say they’re not true viruses are all. They're almost like secretions of the wasp’s body.