Host-pathogen coevolution increases genetic variation in susceptibility to infection.

It is common to find considerable genetic variation in susceptibility to infection in natural populations. We have investigated whether natural selection increases this variation by testing whether host populations show more genetic variation in susceptibility to pathogens that they naturally encounter than novel pathogens. In a large cross-infection experiment involving four species of and four host-specific viruses, we always found greater genetic variation in susceptibility to viruses that had coevolved with their host.