Yellow fever is caused by a type of virus known as a flavivirus. The disease is transmitted by the bite of mosquitoes, typically the Aedes aegypti mosquito.
The mosquito becomes infected by biting an animal (or human) that is already infected with the flavivirus. Infected mosquitoes can then pass it on to other animals or humans that they bite. Once infected, a mosquito is a source of infection throughout its life.
The falvivirus 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 flavivirus 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.
