The world's largest mass yellow fever immunisation has started 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.