Yellow fever is spread through infected mosquitoes and has a wide array of symptoms from nausea and vomiting to kidney failure, jaundice and bleeding.

About half those who develop severe symptoms and are untreated die from the disease - about 30,000 people each year worldwide.

Local health staff in the target countries will vaccinate nearly 12 million people, as well as giving out vitamin A and de-worming tablets.

The World Health Organization says it is not aiming to eliminate yellow fever - there are too many infected mosquitoes in urban areas to make that possible - but to greatly reduce the number of people getting sick with the disease.

It wants to finish mass vaccinations in all high-risk African countries by 2015, but warns there is currently a gap in the funding needed to take the programme to the remaining West African countries most affected by yellow fever.