Ebola first surfaced in 1976, popped up a few times over the next three years, and then virtually disappeared from existence for the next 15 years. But there's no way the virus really disappeared; it had to be hiding somewhere. Where it hid then and continues to hide between outbreaks is a mystery.
What we know: Ebola is a classic 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 to animals like mosquitoes, bats and birds. They have even tested out plants.
So why has it been so hard to find the reservoir species? Well, with the virus hiding for long stretches, there's a good possibility that the master Ebola hoarder is a rare species and hard to find. So scientists have looked. And looked. They've infected a ton of animals and then checked to see if they've survived or looked for antibodies against Ebola. This has been a decades-long quest and still there is no definitive reservoir species.
Recent tests have pointed to fruit bats as the most likely carriers of the disease, but scientists still are unsure how the virus transmits 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.