The Ebola outbreak in West Africa was first reported in March, and has rapidly become the deadliest occurrence of the disease since its discovery in 1976.
More than 1,000 people have died, with Sierra Leona, Guinea and Liberia worst-affected and two deaths in Nigeria, Africa's most populous country.
The World Health Organization (WHO) has declared an International Public Health Emergency.
Researchers from the New England Journal of Medicine have traced the outbreak to a two-year-old girl, who died on 6 December 2013 in a small village in south-eastern Guinea.
Cumulative death toll
The WHO has published updates on the spread of the virus in each of the countries affected.
The figures given are for "confirmed" deaths. They have occasionally been revised down to take account of changes in the countries' reporting methods, for example by excluding "suspected" cases that had previously been included.
2014 outbreak in context
Ebola occurs in regions of sub-Saharan Africa, though normally fewer than 500 cases occur each year. No cases at all were reported between 1979 and 1994.
The 2014 outbreak dwarfs previous epidemics, with WHO figuresindicating that as of 11 August there were 1,975 probable, suspected and confirmed cases, and there had been 1,069 deaths.
The 2014 outbreak as of 4 August. More than 300 people have now died in Sierra Leone, Liberia and Guinea. Source: BBC Online