Yellow fever is a tropical disease, caused by the yellow fever virus, that is spread to humans by infected mosquitoes. The infections are mild, but the disease can cause severe, life-threatening illness.

The are two types of yellow fever: jungle yellow fever and urban yellow fever. The latter is a disease of humans. It is spread by mosquitoes that have been infected by other people. Aedes aegypti is the type of mosquito that usually carries yellow fever from human to human. These mosquitoes have adapted to living among humans in cities, towns, and villages. They breed in discarded tyres, flower pots, oil drums, and water storage containers close to human dwellings. Urban yellow fever is the cause of most yellow fever outbreaks and epidemics.

People get yellow fever from the bite of an infected female mosquito. The mosquito injects the yellow fever virus into the bite.