Sierra Leone has declared a state of emergency and called in troops to enforce an Ebola quarantine, joining Liberia in imposing controls to curb the worst ever outbreak of the virus amid fears it could spread beyond West Africa.

Ebola has killed more than 700 deaths in Liberia, Guinea and Sierra Leone, according to the World Health Organisation. It has also reached Nigeria's biggest city Lagos, where authorities said on Friday a man had died of the virus.

Doctors Without Borders warned on Wednesday that the disease was "out of control".

Ebola's symptoms include internal and external bleeding, vomiting and diarrhoea. The disease kills up to 90 percent of those infected, though the death rate in the current epidemic is about 60 percent.
 
The US and countries in Asia and Europe say they have taken measures to prevent the spread of Ebola in their territories.
 
But what caused the latest outbreak? And how will the world deal with it?
 
Presenter: Adrian Finighan
 
Guests:
 
Dr Ben Neuman: Virologist at the University of Reading.
 
Dr Katie Geary: Medical Director at International SOS.
 
Lewis Brown: Liberian Information Minister

Source: Al Jazeera