CONAKRY: The Ebola virus that has triggered an historic epidemic of hemorrhagic fever is one of the deadliest known to man and has killed almost 1,000 people since January in west Africa.

The World Health Organisation (WHO) launched a global mobilization, declaring the epidemic "a public health emergency of international concern."

It is the "largest, most severe and most complex outbreak in the nearly four-decade history of this disease," WHO director-general Dr Margaret Chan declared.

The latest outbreak has a fatality rate of around 55-60 percent and had killed 961 people and infected around 1,800 by Friday, but for now it has been largely contained to countries in west Africa.

The tropical virus can fell its victims within days, causing severe fever and muscle pain, weakness, vomiting and diarrhea — in some cases shutting down organs and causing unstoppable bleeding.

Prior to the current outbreak, some 1,600 people had been diagnosed with Ebola hemorrhagic fever since the virus was first identified 36 years ago in the Democratic Republic of Congo (then Zaire), and 1,200 had died, the UN health agency says.

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.

People have also 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.

The virus's natural reservoir animal, possibly the bat, is believed to reside in rain forests in Africa and areas of the Western Pacific.

The only large-scale approach for now is to isolate patients and promptly bury the dead, said the WHO.

Hospital staff should use gloves, masks and goggles, and disinfect religiously.

No widely-available medicine or vaccine exists, though an experimental serum has been used under an emergency protocol on two US aid workers.

In the United States the National Institute of Allergy and Infectious Diseases said a vaccine might be available in 2015 for health workers who are putting themselves at extreme risk.