Yellow fever, a mosquito-borne haemorrhagic fever, has an incidence of about 200,000 cases per year in tropical areas of South America and Africa, a small fraction of which are officially reported. The case-fatality rate is 20–50%. There are three clinical periods of the disease — a nonspecific febrile period, a brief period of remission, and a period of intoxication during which the pathognomonic features of severe hepatitis, haemorrhage and renal failure occur. Treatment is supportive. A live attenuated vaccine (17D strain) is widely used to prevent the disease in endemic areas and in travellers. The vaccine elicits immunity within 10 days, and revaccination is recommended every 10 years. Rare neurological adverse events occur, principally in very young infants, in whom the vaccine is contraindicated. A syndrome resembling wild-type yellow fever has been reported at an incidence of about 2.5 per million vaccinations.