Yellow fever and several species of mosquitoes. In cities, it is spread primarily by "Aedes aegypti", a type of mosquito found throughout the tropics and subtropics. The virus is an RNA virus of the genus "Flavivirus". The disease may be difficult to tell apart from other illnesses, especially in the early stages. To confirm a suspected case, blood sample testing with polymerase chain reaction is required. A safe and effective vaccine against yellow fever exists, and some countries require vaccinations for travelers. Other efforts to prevent infection include reducing the population of the transmitting mosquito. In areas where yellow fever is common
Yellow fever vaccine is a small risk of neurologic disease and encephalitis, particularly in individuals with compromised immune systems and very young children. The 17D vaccine is contraindicated in (among others) infants between zero and six months or over 59 years of age, people with thymus disorders associated with abnormal immune cell function, people with primary immunodeficiencies, and anyone with a diminished immune capacity including those taking immunosuppressant drugs. There is a small risk of more severe yellow fever-like disease associated with the vaccine. This reaction, called YEL-AVD, causes a fairly severe disease closely resembling yellow fever caused by virulent strains of the