Fruit bats are the reservoir host of the Ebola virus, while chimpanzees and gorillas are other hosts that become infected as well. The virus was introduced into the human population through contact with blood, organs or secretions of these animals that have already been infected with Ebola.
Ebola is a relatively uncontagious disease as the only way of transmission to animals or humans is either through direct contact with bodily fluids or a bite from an infected host.
The true genetic background of this deadly virus is still unknown, however the disease has most likely mutated from one which only has spikes for bats. Similar to H5N1 (bird flu) and swine flu, this disease has small epidemics every few years before disappearing. The irregular activity of this disease suggests that Ebola was not created by combining with a naturally human disease, but rather a series of mutations of its spikes making strains be able to latch on to human cells.