'Ebola outbreak most challenging ever'
The Ebola outbreak in Guinea has already killed 100 people and crossed into Liberia and Mali.
The virus has already killed more than 100 people and there are fears it could spread beyond the region.
The haemorrhagic virus that kills up to 90 percent of its victims has taken lives in Guinea and crossed borders into Liberia and Mali.
Past outbreaks have been over much smaller areas.
Spread of the Ebola virus
Victims don't show signs of the disease or become infectious for two weeks, sometimes more.
It could be carried by someone boarding a flight out of Guinea, even into Europe and beyond.
Saudi Arabia has stopped granting visas to pilgrims from Guinea while Senegal has closed its borders with its southern neighbour.