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A graph showing a steep spike in the number of cases of yellow fever followed by a decrease in the number of casesVaccination is the single most important measure for preventing yellow fever. Photo credit: WHO Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes. The "yellow" in the name refers to the jaundice that affects some patients. The yellow fever virus is an arbovirus of the flavivirus genus, and the mosquito is the primary vector. It carries the virus from one host to another, primarily between monkeys, from monkeys to humans, and from person-to-person.

Once contracted, the virus incubates in the body for 3 to 6 days, followed by infection that can occur in one or two phases. The first acute phase usually causes fever, muscle pain with prominent backache, headache, shivers, loss of appetite, and nausea or vomiting. Most patients improve and their symptoms disappear after 3 to 4 days.

WHO recommends that every at-risk country have at least one national laboratory where basic yellow fever blood tests can be performed. One confirmed case of yellow fever in an unvaccinated population should be considered an outbreak and a confirmed case in any context must be fully investigated, particularly in any area where most of the population has been vaccinated.