Some 30,000 people die from yellow fever each year
The world's largest mass yellow fever immunisation is starting in West Africa, targeting 12 million people.
Health charities are rolling out the vaccines to people in Benin, Liberia and Sierra Leone.
Thirteen African nations are said to be at high risk of outbreaks of yellow fever, which can kill if untreated.
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.