It causes characteristic symptoms and a semi-predictable outcome. In the case of Ebola virus, the rarity and the geographic isolation of outbreaks have left major questions about its natural history unanswered. Key among these is what harbors it between epidemics. Viruses are not free-living organisms, capable of capturing food and water from the environment and multiplying on their own. Instead, they are packets of genetic instructions that must invade living cells and use the latter's far more complicated machinery to replicate themselves and, ultimately, spread to new hosts.
Some viruses can survive only in human beings.
Ebola, however, clearly has some nonhuman reservoir. The tropics seem to be its home range. The first epidemic, like the latest, occurred in Zaire, where 318 persons became ill in 1976 and 88 percent died. Ticks, spiders, bats and several small animals have been investigated as the source of Ebola virus, without success. Monkeys definitely get the disease but because they often die from it, they are an unlikely site for the virus's long-term residence.