The first cases of Ebola virus disease were seen in 1976 in southern Sudan and northern Zaire, located 500 miles apart from eachother. More cases occurred in Zaire in 1977 and in Sudan in 1979. The virus was mostly dormant after the initial outbreaks -- however, in 1989, a subtype of the virus caused a scare in Reston, Virginia, when four laboratory workers became infected from cynomolgus monkeys imported from the Philippines. (The Ebola "virus" is actually a group of four subtypes that are 30% to 45% different at the nucleotide level, suggesting that there are four different types of ebola viruses.)

Nearly twenty years later, in May 1995, a deadly outbreak appeared in Zaire. Although the source of the outbreak is not quite known even today, it is suspected that the outbreak began when a laboratory worker became infected from a monkey. Just as the outbreak in Zaire was beginning to subside, the World Health Organization (WHO) reported 20 cases of Ebola in Gabon (West Africa), sparking new concerns about the disease.

The natural history of Ebola itself is still a mystery -- no one knows where the virus lays dormant between epidemics or how it maintains its survival in nature. Although the Reston outbreak suggested that the cynomolgus monkeys could be a reservoir, there are several other possible reserviors, such as other monkeys, bats, and insects.