The tropical virus can fell its victims within days, causing severe fever and muscle pain, weakness, vomiting and diarrhoea -- in some cases shutting down organs and causing unstoppable bleeding.
No medicine or vaccine exists.
Of about 1,850 people diagnosed with Ebola haemorrhagic fever since the virus was first identified 36 years ago in the Democratic Republic of Congo (then Zaire), 1,200 have died, the UN health agency says.
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Experts say the virus, despite being extremely virulent, is containable because it kills its victims faster than it can spread to new ones.
The incubation period between exposure and the first symptoms varies from two to 21 days.
There are five species of the virus, of which three are particularly dangerous with fatality rates from 25 to 90 percent, according to the WHO.
It is transmitted through contact with the blood, body fluids, secretions or organs of an infected person.
The virus has been known to spread at burials where mourners touch the body, but doctors and nurses have also fallen ill after failing to take adequate precautions.
Even testing blood specimens for the disease presents "an extreme biohazard risk", states the WHO, and is done only in the strictest containment conditions.
People have contracted the virus after handling infected chimpanzees, gorillas, monkeys, forest antelope and porcupines, dead or alive, in the Ivory Coast, Congo and Gabon.
Named after a small river in DR Congo, the virus killed 37 people in western Uganda in 2007 and at least 170 in the north of the country in 2000.
In a new outbreak, 14 people had died in the last three weeks -- including one in the capital Kampala.
The only approach for the moment is to isolate patients and promptly bury the dead, said the WHO.
Hospital staff should use gloves, masks and goggles, and disinfect religiously.
"Several potential vaccines are being tested but it could be several years before any is available," says a WHO factsheet.
"A new drug therapy has shown some promise in laboratory studies and is currently being evaluated."
The virus's natural reservoir animal, possibly the bat, is believed to reside in rain forests in Africa and areas of the Western Pacific.
