Ebola hemorrhagic fever is a disease caused by one of five different Ebola viruses. Four of the strains can cause severe illness in humans and animals. The fifth, Reston virus, has caused illness in some animals, but not in humans.

The first human outbreaks occurred in 1976, one in northern Zaire (now Democratic Republic of the Congo) in central Africa: and the other, in southern Sudan (now South Sudan). The virus is named after the Ebola River, where the virus was first recognized in 1976, according to the Centers for Disease Control and Prevention (CDC).

Ebola is extremely infectious but not extremely contagious. It is infectious, because an infinitesimally small amount can cause illness. Laboratory experiments on nonhuman primates suggest that even a single virus may be enough to trigger a fatal infection.
Ebola could be considered moderately contagious because the virus is not transmitted through the air.
Humans can be infected by other humans if they come in contact with body fluids from an infected person or contaminated objects from infected persons. Humans can also be exposed to the virus, for example, by butchering infected animals.

While the exact reservoir of Ebola viruses is still unknown, researchers believe the most likely natural hosts are fruit bats.

Symptoms of Ebola typically include: weakness, fever, aches, diarrhea, vomiting and stomach pain. Additional experiences include rash, red eyes, chest pain, throat soreness, difficulty breathing or swallowing and bleeding (including internal).
Typically, symptoms appear 8 to 10 days after exposure to the virus, but the incubation period can span 2 to 21 days.

Unprotected health care workers are susceptible to infection because of their close contact with patients during treatment.
Ebola is not transmissible if someone is asymptomatic and usually not after someone has recovered from it. However, the virus has been found in semen for up to three months, and "possibly" is transmitted from contact with that semen, according to the CDC.
Deadly human Ebola outbreaks have been confirmed in the following countries: Democratic Republic of the Congo (DRC), Gabon, South Sudan, Ivory Coast, Uganda, Republic of the Congo (ROC), Guinea, Liberia and Sierra Leone.
There are five subspecies of the Ebola virus: Zaire ebolavirus (EBOV), Bundibugyo ebolavirus (BDBV), Sudan ebolavirus (SUDV), Taï Forest ebolavirus (TAFV) and Reston ebolavirus (RESTV).
March 25, 2014 - The CDC issues its initial announcement on an outbreak in Guinea, and reports of cases in Liberia and Sierra Leone. "In Guinea, a total of 86 suspected cases, including 59 deaths (case fatality ratio: 68.5%), had been reported as of March 24, 2014. Preliminary results from the Pasteur Institute in Lyon, France suggest Zaire ebolavirus as the causative agent."
January 18, 2015 - Mali is declared Ebola free after no new cases in 42 days.
May 9, 2015 - WHO declares an end to the Ebola outbreak in Liberia. More than 4,000 died.
December 29, 2015 - WHO declares Guinea is free of Ebola after 42 days pass since the last person confirmed to have the virus was tested negative for a second time.
January 15, 2016 - A new case of Ebola in Sierra Leone, in which the patient died, is confirmed by WHO and CDC.
1990 - In Texas and Virginia quarantine facilities, four humans develop Ebola antibodies after contact with monkeys imported from the Philippines. None of the humans has symptoms.

August 1, 2018 - The DRC's Ministry of Health declares an Ebola virus outbreak in five health zones in North Kivu province and one health zone in Ituri province. Containment of the virus and response may be difficult due to geographic and political challenges. As of August 6, a total of 43 cases have been reported, including 36 deaths.