The Ebola virus now threatening Sierra Leone is one of the deadliest known to man, with more than 100 confirmed deaths from the disease in West Africa since January.
According to a statement by the World Health Organisation (WHO), 258 cases of viral haemorrhagic fever, of which 174 have resulted in deaths, have been reported in the Republic of Guinea.
Out of the 258 cases, the Guinean government has said that 146 were confirmed Ebola cases, 95 of them deadly.
Ebola has also been reported in neighbouring Liberia and Sierra Leone, with a total of 10 confirmed deaths, including one reported on 26 May, out of more than a dozen cases.
No medicine or vaccine exists for the virus, which is named after a small river in the Democratic Republic of Congo.
Since the first case of Ebola was identified 38 years ago in the Democratic Republic of Congo (then Zaire), the virus has killed around 1 400 people.
Experts say that although it is extremely virulent, the virus can be contained 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%, according to the WHO.
It is transmitted through contact with the blood, body fluids, secretions or organs of an infected person.
Extreme biohazard risk
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.
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.