In some cases, an alliance between different infections can actually work to a host’s advantage. The pea aphid carries guardian bacteria called Hamiltonella defensa that protect their host from parasitic wasp. If the wasp lays an egg inside the aphid, the bacteria kill the wasp larvae. But H.defensa only works when it itself is infected by a virus called APSE, which produces the toxins that kill the wasps. Faced with an enemy that threatens them all, the virus, the bacterium and the aphid have formed an evolutionary alliance, with infection as its foundation.