Marburg and virulent Ebola viruses are maintained in hosts that are rare and have little contact with humans or do not readily transmit virus. Bats (particularly solitary microchiropteran species) are leading contenders as reservoir hosts. Virus transfer to humans occurs by contact with the primary reservoir or via an intermediate animal that acquired infection from the reservoir and is, in turn, hunted by humans. An interesting possibility is that filoviruses may be arthropod or plant viruses, with non-blood-feeding arthropods transmitting the virus to intermediate hosts or humans during oral ingestion or envenomation. Paradoxically, in Africa, Ebola virus disease has high lethality and high seroprevalence as determined by the IFA test. If the seroreactivity is confirmed by more specific tests, then the Ebola virus serogroup in Africa probably contains an antigenically cross-reactive, enzootic, nonpathogenic agent(s). Such viruses may have separate life cycles or may give rise to virulent strains by mutation.