The Cause of Ebola
Researchers have hypothesized that the first patient with Ebola becomes infected with the virus through contact with an infected animal; however, just as scientists are unsure of the animal host for the virus, they are also unsure how an Ebola outbreak occurs.
No case of Ebola infection in humans has ever been reported in the United States. Ebola-Reston virus caused severe illness and death in monkeys imported to research facilities in the United States and Italy from the Philippines; during these outbreaks, several research workers became infected with the virus, but did not become ill.
Once Ebola virus infects the first human, scientists do know how it is spread from human to human. Transmission occurs through direct contact with someone who has Ebola, or their body fluids (such as blood or secretions). The late stage of an infection is the most common time for the spread of the virus to occur.
When someone becomes infected with Ebola, he or she will not feel sick immediately. Although the virus is multiplying inside his or her body, the person typically feels normal for about 2 to 21 days after being infected. This period between transmission of the virus and the beginning of Ebola symptoms is called the incubation period.