Fig. 1. Cheating occurs throughout the natural world, including in viruses.

a The common cuckoo (Cuculus canorus) lays eggs in other birds’ nests, here tricking a reed warbler (Acrocephalus scirpaceus) into taking care of a much larger cuckoo chick^21,143. b Cells of the bacterial pathogen Pseudomonas aeruginosa which do not produce iron-scavenging molecules (labelled in green) are able to exploit those produced by others (labelled in white), and consequently grow much larger colonies⁵. c, d In Vesicular Stomatitis Virus (VSV), when a defective interfering genome (labelled in green) is grown in a mixed infection with wild-type VSV (labelled in red), the defective interfering genome exploits replicase proteins encoded by the wild-type cooperator, resulting in a colony (c) that is dominated by the defective interfering genome, and grows less effectively than a colony consisting just of the cooperative wild-type (d)¹⁴⁴. e, f In cucumber mosaic virus (CMV) infections, a satellite (satCMV) exploits gene products encoded by the wild-type, substantially reducing the overall viral load and leading to less severe infections in plants infected by both satellite and wild type (e) compared to plants infected by just the wild type (f)¹⁴⁵. Panel a from Per Harald Olsen (CC BY-SA 3.0), no changes made, reference ¹⁴⁶. Panel b from Melanie Ghoul, no changes made, reference ⁵. (c and d) from John Yin, no changes made, ref. ¹⁴³. e, f from Zhiyou Du, no changes made, ref. ¹⁴⁴.