By Helen Briggs
Scientists have tracked the spread of Ebola in West Africa, revealing genetic clues to the course of the outbreak.
Genetic analysis of patient samples suggests the virus spread from Guinea to Sierra Leone at a single funeral.
The virus is mutating and must be contained rapidly, warn African and US experts. But they say there is no evidence the virus is changing its behaviour.
The current outbreak is the largest ever, with more than 3,000 cases.
The number of cases could exceed 20,000 before the outbreak is stemmed, according to the World Health Organization.