Reticuloendotheliosis viruses (REVs) are unusual retroviruses circulating in birds; they are not related to any other avian retroviruses, but instead resemble endogenous viral elements (EVEs) found in mongooses. Genetic analysis now reveals that both REVs and EVEs are derived from a mammalian retrovirus that infected the ancestor of mongooses more than 8 million years ago. Cross-species transmission to birds, however, was a very recent event and involved the accidental spread of REVs during experimental studies. In the 1930s, researchers isolated Plasmodium lophurae from a pheasant in the Bronx Zoo (New York, USA), where the bird lived in close proximity to exotic animals, which might have carried REVs. In search of an animal model of malaria, the researchers inoculated numerous bird species not only with P. lophurae, but also, by accident, with REVs contaminating the parasite stocks. Subsequently, REVs integrated into the genome of two large avian DNA viruses, fowlpox virus and gallid herpesvirus type 2, which causes Marek's disease. Global vaccination campaigns with attenuated strains of these viruses allowed REVs to spread even further. National Geographic/PLoS Biol.