The exact origin, locations, and natural habitat (known as the natural reservoir) of the virus remain unknown; however, on the basis of available evidence and the nature of similar viruses, researchers believe that the Ebola virus lives in an animal host that is native to the African continent. They continue to search for the exact animal.
Just as scientists are unsure of the animal host for the virus, they are also unsure how an outbreak occurs. Researchers have hypothesized that the first patient with Ebola becomes infected with the virus through contact with an infected animal.
Once the virus infects the first human, transmission of Ebola continues through direct contact with an infected person or his or her body fluids (such as blood or secretions). The spread of the virus from an infected person most often occurs during the late stages of an infection.
(Click Transmission of Ebola for a closer look at how the disease is transmitted.)
When a person becomes infected with Ebola virus, it begins to multiply within the body. After four to six days, on average, symptoms can begin. The period between infection with the virus and the start of symptoms is called the incubation period. The incubation period for Ebola can be as short as 2 days or as long as 21 days.
(Click Ebola Incubation Period for more information.)
Ebola symptoms usually start suddenly. Symptoms of this disease can include:
- Sore throat
- Severe headache
- Joint and muscle aches
- Dry, hacking cough
- Stomach pain
- Internal bleeding
- External bleeding
(Click Ebola Symptoms for more information on possible signs and symptoms of this illness.)