Parasitic wasps make a living by snatching the bodies of other insects and using them as living incubators for their grubs. Some species target caterpillars, and subdue them with a biological weapon. They inject the victim with “virus-like particles” called polydnaviruses (PDVs), which weaken its immune system and leave the wasp grub to develop unopposed. Without the infection, the wasp egg would be surrounded by blood cells and killed.

The wasps’ partners in body-snatching are very different to all other viruses. Once they have infected other cells, they never use the opportunity to make more copies of themselves. They actually can’t. To complete their life cycles, viruses need to package their genetic material within a coat made of proteins. In most