From Nzerekore, a remote area of south-eastern Guinea, the virus has spread to the capital, Conakry, and neighbouring Liberia and Sierra Leone.
A man who flew from Liberia to Lagos in July was quarantined on his arrival and later died of Ebola - the first case in Nigeria. One of the nurses who treated him and an official who came into direct contact with him have since died.
The medical charity Medecins Sans Frontieres (MSF) says the outbreak is "unprecedented" in the way the cases were scattered in multiple locations across Guinea, hundreds of kilometres apart, and says it is a "race against time" to check people who come into contact with sick people.