The virus kills up to nine out of every 10 people who get it - but until now, Ebola has been confined to isolated outbreaks in remote African villages.
The appearance of the virus in Conakry, the capital of Guinea, is a major worry. There is no cure and no vaccine.
Dr Ben Neuman, a virologist at the University of Reading, says: "Ebola virus is one of the things that keep public health officials up at night. If this virus spread between people more easily, it would probably be more deadly than the black plague."
So why is so little known about Ebola? And what are the scientists doing to try to stop it?
Presenter: Mike Hanna
Panu Saaristo, a team leader in Conakry with the International Federation of Red Cross and Red Crescent Societies
Ben Neuman, a virologist at the University of Reading in the UK
Meinie Nicolai, a nurse and field worker with Doctors Without Borders
Hugh Pennington, a professor of bacteriology at the University of Aberdeen