By Thomas Moore, Health and Science Correspondent
Scientists have confirmed that the Ebola virus is mutating - and could become more contagious.
Researchers at the Institute Pasteur in France say they have detected substantial changes to the genetic code of the virus.
Although they don't yet know what effect that will have on the epidemic, they fear the virus could spread from person to person more easily.
In previous Ebola outbreaks, the virus has only infected a few dozen people and died out within weeks.
But the current prolonged epidemic has given the virus time to pass through many more generations, in many more people, giving it greater opportunity to evolve.
Scientists say it is highly unlikely that the mutations will lead to the Ebola becoming airborne.
But the virus - originally from fruit bats - could become better adapted to living in humans.
Scientists say some people may even carry the virus without showing symptoms, leading it to spread more rapidly.
The changes to the virus could also result in resistance to drug treatments - and reduce the effectiveness of vaccines currently being developed.
The latest figures from the World Health Organisation show 22,000 people have been infected by the virus and 8,800 have died.
But the infection rate in recent weeks is just one sixth of what it was in October.
UN Ebola coordinator David Nabarro said: "The number of cases is decreasing week by week and getting to zero in many places.
"But we still see occasional flare-ups. The epidemic is not contained yet."
New results show that a vaccine tested on volunteers in Oxford successfully triggers an immune response, but at a lower level than had been hoped for.
Scientists are now studying the effect of a booster dose in the hope that it will give better protection.