Can US 'secret serum' stop Ebola outbreak?

On Friday, the World Health Organisation (WHO) declared the West African outbreak of Ebola an "international emergency".

In the US, two Americans who contracted the virus are being treated with a secret serum, and are understood to be recovering in Atlanta.

The serum is controversial because it has not been tested on humans before - and because it is not being given to others in Africa who are suffering from the virus.

Alastair Leithead visited San Diego where the serum was developed and spoke with researchers working on finding a cure.