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.