It is thought that parasites may exert selective pressure on the social structure of host populations. We compared the impact of feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV), two retroviruses commonly found in domestic cats (Felis catus). Because of low transmissibility and virulence, both infections have a worldwide distribution and low prevalence. Transmission modes differ: FIV is transmitted only through biting, while FeLV transmission occurs by biting, licking, grooming, and sharing food and from mother to fetus. FeLV is also more pathogenic than FIV. We compared FIV and FeLV prevalence and risk factors within five populations of cats. FIV infection occurred almost exclusively among adult male cats fighting to acquire and maintain dominant status. Classes at risk for FeLV infection included sexually intact cats allowed to roam freely. The impact of FeLV on host population growth was greater than that of FIV but varied among populations. Our results show that FIV is favoured by individual aggressiveness and a hierarchical social system, while FeLV is more prevalent among socially active cats. FeLV may constitute a source of selective pressure against numerous amicable contacts, particularly in urban cat populations, where aggression among individuals is reduced.