Retroviruses are viruses that multiply by incorporating their genes into the genome of a host cell. If the infected cell is a germ cell, the retrovirus can then be passed on to the next generation as an ‘endogenous’ retrovirus (ERV) and spread as part of the host genome in that host species. In vertebrates, ERVs are ubiquitous and sometimes make up 10 per cent of the host genome. However, most retrovirus integrations are very old, already degraded and therefore inactive — their initial impact on host health has been minimized by millions of years of evolution. A research team has now discovered a recent case of retrovirus colonization in a rodent from New Guinea, the white-bellied mosaic-tailed rat.