Fig. 2. Viruses can cooperate when infecting the same cell, and also when infecting different cells.

a In coinfection, shared viral gene products, such as replicase enzymes or capsid proteins, have the potential to benefit other viral genomes, and hence act as cooperative ‘public goods’ (Box 2). b When multiple viral genomes infect a host cell, there is also the potential for cheating, where some individuals benefit from the public good without producing it (cheats). c Cooperation can also occur between viral genomes that infect different host cells. Here, cooperative viruses produce a gene product that prevents host cells releasing interferon, keeping the local population of host cells susceptible to infection, and hence providing a benefit to other viral genomes infecting different cells⁵. d Cheat viruses do not block interferon, and hence replicate faster than cooperators. This cheating is costly for the viral population as a whole, because interferon is released from the infected cell, binding to nearby cells, which become resistant to infection by other viral genomes.