Some parasitic wasps lay eggs in caterpillars, and they subdue their victims with a biological weapon. They inject the victim with “virus-like particles” called polydnaviruses (PDVs) that weaken its immune system and allows the wasp egg to develop unharmed. This is an extreme alliance. The genes that make the viruses’ coats are actually found inside the genome of the wasp. Neither partner can reproduce without the other.