Viruses and bacteria often act as parasites, infecting a host, reproducing at its expense and causing disease and death. But not always – sometimes, their infections are positively beneficial and on rare occasions, they can actually defend their hosts from parasitism rather than playing the role themselves.

In the body of one species of aphid, a bacterium and a virus have formed a unlikely partnership to defend their host from a lethal wasp called Aphidius ervi. The wasp turns aphids into living larders for its larvae, laying eggs inside unfortunate animals that are eventually eaten from the inside out. But the pea aphid (Acyrthosiphon pisum) has a defence – some individuals are infected by guardian bacteria (Hamiltonella defensa) that save their