virus of the family Filoviridae that is responsible for a severe and often fatal viral hemorrhagic fever; outbreaks in primates such as gorillas and chimpanzees as well as humans have been recorded. The disease is characterized by extreme fever, rash, and profuse hemorrhaging. In humans, fatality rates range from 50 to 90 percent.

The virus takes its name from the Ebola River in the northern Congo basin of central Africa, where it first emerged in 1976. Outbreaks that year in Zaire (now Congo 【Kinshasa (Congo)】) and The Sudan (Sudan, history of the) resulted in more than 400 deaths. A subsequent outbreak in Congo (Kinshasa) in May 1995 prompted temporary quarantine of the Kikwit region, and more than 250 people died. Later outbreaks in Uganda in 2000 and in Congo (Kinshasa) in 2002 also resulted in several hundred deaths. In September 2007 an outbreak was confirmed in Congo (Kinshasa) in the Kasai-Occidental (West Kasai) province, located in the south-central region of the country. However, while Ebola was detected in blood samples from some people that fell ill, other people were found to be infected with Shigella, the bacterium that causes dysentery—a disease whose symptoms are similar to the early symptoms of Ebola. As a result, although several hundred people became ill and more than 160 people died during the Ebola outbreak, it was unclear how many of the deaths were actually caused by Ebola. Less than two years later, in December 2008, a second outbreak of the disease was confirmed in the Kasai-Occidental province. Ebola had been detected in just four people by early 2009; however, another 42 cases were suspected, and some 200 people were under close observation for infection. Although 13 deaths had been reported in association with the outbreak, samples collected from the victims did not test positive for Ebola.

Ebola is closely related to the Marburg virus, which was discovered in 1967, and the two are the only members of the Filoviridae that cause epidemic human disease. Four strains of Ebola virus, known as Ebola-Zaire, Ebola-Sudan, Ebola–Côte d'Ivoire, and Ebola-Reston, named for their outbreak locations, have been described. An unnamed fifth strain was identified in November 2007 in an outbreak near the border of Uganda and Congo (Kinshasa).

Ebola-Zaire causes death in 80 to 90 percent of cases, and Ebola-Sudan causes death in 50 percent of cases. Ebola–Côte d'Ivoire, found in dead chimpanzees in the Taï National Park in southwestern Côte d'Ivoire, can infect humans, although only two human cases have been documented, and both individuals survived. Ebola-Reston, which was originally discovered in laboratory monkeys in Reston, Va., in 1989, was also detected in laboratory monkeys in other locations in the United States in 1990 and 1996, as well as in Siena, Italy, in 1992. All the monkeys infected with Ebola-Reston have been traced to one export facility located in the Philippines, although the origin of the strain has not been identified. Similar to Ebola–Côte d'Ivoire, Ebola-Reston does not appear to cause death in humans. The unnamed strain discovered in 2007 causes death in about 25 percent of cases.

Viewed through an electron microscope, the Ebola virus appears as long filaments, sometimes branched or intertwined. The virion (virus particle) contains one molecule of noninfectious, single-stranded RNA (ribonucleic acid). It is not known how the Ebola virus attacks cells; however, it has been postulated that the virus produces proteins that suppress the immune system, allowing reproduction of the virus to continue unhindered. Viral hemorrhagic fevers similar to Ebola typically are carried by arthropods and rodents; however, the natural reservoir for the Ebola virus has yet to be discovered. Among the suspected reservoirs for Ebola are bats, primates, rodents, and insects that inhabit tropical forests in Africa and Asia. Ebola can be transmitted through contact with infected blood, bodily fluids, and possibly respiratory secretions. The virus has also been detected in the organs of patients after recovery from the fever. Unsanitary conditions and lack of adequate medical supplies may be factors in the spread of the disease.

The Ebola virus has an incubation period of 4 to 16 days. The onset is sudden and harsh. Infected persons develop fever, severe headaches and muscle aches, and loss of appetite. Within a few days the virus causes a condition known as disseminated intravascular coagulation, which is marked by both blood clots and hemorrhaging. In the case of Ebola fever, clots are concentrated in the liver, spleen, brain, and other internal organs, forcing capillaries to bleed into surrounding tissue. Nausea, vomiting and diarrhea with blood and mucus, conjunctivitis, and sore throat soon follow. A maculopapular rash (discoloured elevations of the skin) appears on the trunk and quickly spreads to the limbs and head. The patient is then beset by spontaneous bleeding from body orifices and any breaks in the skin, such as injection sites, and within the gastrointestinal tract, skin, and internal organs. Death is usually brought on by hemorrhaging, shock, or renal failure and occurs within 8 to 17 days.

There is no known treatment for Ebola fever, although immune plasma may be beneficial. Current therapy consists of maintenance of fluid and electrolyte balance and administration of blood and plasma to control bleeding. The spread of the virus can be contained by barrier nursing, handling of infected blood and tissue in isolated laboratory units, and proper decontamination of reusable equipment.