The simian orthopoxvirus, also known as the monkeypox virus, circulates enzootically in the rainforests of Central and West Africa; it can be transmitted to humans, causing a syndrome with clinical manifestations similar to those of smallpox... From February to August 1996, a total of 71 clinical cases of simian orthopoxvirus infection, including 6 fatalities, were reported in 13 villages in the Karato-Kombe health zone, Sankuru subregion, East Kasai region (Zaire)... This report summarizes the initial findings of a multidisciplinary investigation: in the majority of cases studied in 1996 and 1997, human-to-human transmission appears to have occurred; in contrast, in previous years, the reported cases were primarily sporadic cases resulting from animal-to-human transmission. (adapted from the text).