Nineteen arenaviruses have been isolated in the Old and the New World, most of them identified in South or Central America. Current evidence suggests that arenaviruses have co-evolved along with their rodent hosts over a time scale of as much as 9 million years. Among six arenaviruses so far known to cause human illness, only one, Lassa virus, is from Africa. Three other arenaviruses have been isolated in Africa: Mopeia virus from Mastomys natalensis in southern Africa (Wulff et al. 1977; Johnson et al. 1981a) and two other viruses from two different rodent species, Arvicanthus and Praomys (Gonzalez et al. 1983; Swanepoel et al. 1985) in the Central African Republic. None of them has been associated with human illness (Georges et al. 1985). Lassa fever was described as a clinical entity in the 1950s, well before the Lassa virus was identified in 1969, when it was isolated from febrile American missionaries working in northern Nigeria (Frame et al. 1970; Buckley and Casals 1970). Further expansion of the family is to be anticipated with increased efforts at surveillance, including a possible a new North American arenavirus capable of causing fatal human disease.