Ebola virus: The challenge of tackling the virus
30 July 2014 Last updated at 23:50 BST
Scientists have been aware of Ebola virus since the mid-1970s, but so far no vaccine or cure has been developed.
Ebola kills up to 90% of those infected, but patients have a better chance of survival if they receive early treatment.
It spreads through contact with an infected person's bodily fluids.
The outbreak - the world's deadliest to date - was first reported in Guinea in February. It then spread to Liberia and Sierra Leone.
David Shukman reports.