Ebola outbreaks are extremely rare. There were fewer than 30 reported outbreaks between 1976, when the virus was discovered for the first time, and 2010. None of these outbreaks affected more than 1,000 individuals. Outbreaks of the disease tend to be self-limiting because of the extremely high mortality rate associated with ebola infection. Many cases of the disease have occurred, not in normal human populations, but within healthcare settings, where limited supplies of sterile goods resulted in one patient infecting another patient or a healthcare worker.
The virus that causes ebola outbreaks is thought to survive primarily in animal reservoirs. Four of the five known strains of the virus exist primarily within the jungles of central Africa, where they presumably have established themselves within animal populations, most likely primates, without killing those populations. A strain of this virus also exists in animal reservoirs, including some pigs, in the Philippines. Outbreaks of the disease occur when the virus jumps from that animal reservoir into the human population and begins to spread from person to person.
Ebola hemorrhagic fever is an extremely dangerous disease that causes a variety of symptoms, including bleeding, diarrhea, and vomiting. These are all symptoms that can leave secretions in the environment, which can, in turn, carry the virus to new human hosts. This is the primary vector for the transmission of the ebola virus. Ebola is not known to spread as an airborne pathogen, which greatly limits its ability to travel from human host to human host. If the disease acquired such a capability through mutation, ebola outbreaks would be much more dangerous, as it can take up to three weeks before symptoms appear, a period during which patients could still transmit the disease.
Though ebola outbreaks have largely been confined to central Africa, a small number of researchers in Western countries have been exposed to the virus during the course of work with laboratory animals. Of the few researchers exposed, none developed symptoms of the disease, although four did become infected with the disease. Standard animal quarantine measures have proven to be highly effective in limiting the spread of the disease.
An additional factor that reduces the frequency of ebola outbreaks is the virus' inability to establish itself permanently in human carriers. Humans infected with ebola may well die as a result of the disease. A patient who survives a bout of ebola, however, will be free of infection, as the immune system can rid the body of the virus.