The Ebola virus plaguing west Africa, which has killed at least 660 since the worst-ever outbreak began there in January, is one of the deadliest known to man and can kill victims within days.
Africa's most populous country Nigeria on Friday became the fourth west African country to be affected by the virus, announcing that a Liberian national in guarantine in a Lagos hospital had died of the disease.
New data from the World Health Organisation released on Friday, and dating from July 20, - before the announcement of the death in Nigeria - said the death toll had risen to 660.
The UN health agency said the number of cases of Ebola, first identified 38 years ago in the Democratic Republic of Congo, then Zaire, had risen to 1 093.
It said 28 news deaths were recorded between July 18 and July 20. Thirteen were in Sierra Leone, 11 in Liberia and four in Guinea, which had previously borne the brunt.
Forty-five new cases were recorded over the same period.
Although Guinea recorded the lowest number of new cases - five - it still has the highest death toll.
In total, Guinea has seen 314 fatalities and 415 cases since the outbreak began in January.
Sierra Leone's case-count has now overtaken Guinea's, however. It reported 12 new cases, taking its total to 454, with 219 deaths.
Liberia reported 28 new cases, lifting its total to 224. Of those, 127 have been fatal.
Ebola is one of several viruses responsible for haemorrhagic fever.
No medicine or vaccine exists for the tropical virus, named after a small river in the DR Congo.
Five “species” of Ebola have been identified so far, and have been named Bundibugyo, Sudan, Zaire, Tai Forest and Reston.
The first three are particularly dangerous, with fatality rates of up to 90 percent.
The Reston species has also been identified in China and the Philippines, but no associated deaths have been reported in those countries to date.
Ebola causes severe fever and muscle pain, weakness, vomiting and diarrhoea Ä in some cases shutting down organs and causing unstoppable bleeding.
It is a so-called filovirus, transmitted through contact with the blood, body fluids, secretions or organs of an infected person.
Experts say that while extremely virulent, the virus can be contained because it kills victims faster than it spreads.
The incubation period between exposure and the first symptoms varies from two to 21 days.
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 risk”, the WHO has warned, and is done only in the strictest containment conditions.
The virus's natural host in Africa is thought to be a species of rainforest bat, while another concentration has been found in the western Pacific region.
People have contracted the virus after handling both dead and living chimpanzees, gorillas, monkeys, forest antelope and porcupines.
For now, the only approach is to isolate patients and promptly bury the dead, says 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 are available,” according to a WHO factsheet.
“A new drug therapy has shown some promise in laboratory studies and is currently being evaluated.” - Sapa-AFP