Ebola first surfaced in 1976, popped up in isolated places over the next several years, and then virtually disappeared until a 1995 outbreak in the Congo that killed 250 people. Five years later, an outbreak in Uganda killed 224 more people. Since then, it's continued to return on an irregular basis, with a 2014 outbreak in the Congo taking 49 lives [source: CDC].
What we know: Ebola seems to be a zoonotic disease, meaning that it occurs naturally in animals but can be transmitted to humans. So that means a likely resting place, or reservoir, for Ebola could be an animal that becomes the source of direct transmission. But here's the tricky part: Since Ebola lies in silence for long periods, animals that get sick from the disease, like primates, are probably not the reservoirs. If a monkey were the reservoir, that would mean that the virus would be able to hang out in the monkey for long stretches of time without making the monkey sick. And we know that monkeys are super susceptible to the disease, so scientists have had to look elsewhere [sources: Rewar, etal.].
Researchers have injected the Ebola virus into a variety of animals — from pigeons to lizards — and then checked to see if they've survived or looked for antibodies against Ebola [source: Cowart].
Recent research has pointed to fruit bats as likely reservoirs of Ebola, though not all scientists agree. But it's still unclear just how the virus would make the jump from the bats to humans and nonhuman primates. We only know that some of these African bats can support the virus replicating in their bodies without getting sick and that bats have been associated with known index cases (the first human infected in an outbreak) in past outbreaks [sources: Kupferschmidt, Cowart, Vogel].