Yellow fever is caused by a type of virus known as a flavivirus. The disease is transmitted by the bite of certain mosquitoes (see below).
Yellow fever can't be spread by close contact between two people.
How yellow fever is spread
In urban and some rural areas, it's usually the Aedes aegypti mosquito that spreads yellow fever. However, other types of mosquitoes may carry the virus in forested areas. The mosquitoes usually bite during daylight hours.
The mosquito becomes infected by biting a monkey or human that's already infected with the virus. Infected mosquitoes can then pass the disease on to other monkeys or humans that they bite. Once infected, a mosquito is a source of infection throughout its life.
The virus is thought to be widespread among monkeys that live in the jungle canopy (the tree tops) of some parts of Africa and the Americas.
Occasionally, an infected mosquito will pass the virus on to a person in the jungle, such as a forestry worker, who may then become a source of infection when they return to their community.
The risk of an urban yellow fever outbreak is highest in areas close to jungles where the mosquitoes and infected monkeys live.
Who's at risk?
If you're travelling, your risk of getting yellow fever may depend on:
