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Yellow fever (YF) is a viral haemorrhagic fever transmitted by infected mosquitoes.
Yellow fever can be recognized from historic texts stretching back 400 years. Infection causes a wide spectrum of disease, from mild symptoms to severe illness and death. The "yellow" in the name is explained by the jaundice that affects some patients, causing yellow eyes and yellow skin.
There are three types of transmission cycle: sylvatic, intermediate and urban. All three cycles exist in Africa, but in South America, only sylvatic and urban yellow fever occur.
Sylvatic (or jungle) yellow fever occurs in tropical rainforests where monkeys, infected by sylvatic mosquitoes, pass the virus onto other mosquitoes that feed on them; these mosquitoes, in turn bite and infect humans entering the forest. This produces sporadic cases, the majority of which are often young men working in the forest e.g. logging.
The intermediate cycle of yellow fever transmission occurs in humid or semi-humid savannahs of Africa, and can produce small-scale epidemics in rural villages. Semi-domestic mosquitoes infect both monkey and human hosts and increased contact between man and infected mosquito leads to disease. This is the most common type of outbreak seen in recent decades in Africa.
Urban yellow fever results in large explosive epidemics when travellers from rural areas introduce the virus into areas with high human population density. Domestic mosquitoes, most notably Aedes aegypti, carry the virus from person to person. These outbreaks tend to spread outwards from one source to cover a wide area.
Yellow fever can be prevented by vaccination. In order to protect people living in areas at high risk of yellow fever transmission, WHO's dual strategy for prevention of yellow fever epidemics relies on preventive mass immunization campaigns followed by infant routine immunization.
Yellow fever causes epidemics that can affect 20% of the population. When epidemics occur in unvaccinated populations, case-fatality rates may exceed 50%. No treatment beyond supportive care exists.