Yellow fever is a serious viral infection that is usually spread by a type of mosquito known as the Aedes aegypti mosquito. The virus is a 40 to 50 nm enveloped positive-sense RNA virus, the first human virus discovered and the namesake of the Flavivirus genus. Yellow fever mainly occurs in sub-Saharan Africa (countries to the south of the Sahara desert), South America and in parts of the Caribbean.
There are two forms of yellow fever: Urban and Jungle. Clinically and etiologically they are identical.
Urban yellow fever is an epidemic viral disease of humans transmitted from infected to susceptible persons by Aedes aegypti mosquitoes, which breed in domestic and peri domestic containers (e.g., water jars, barrels, drums, tires, or tin cans) and thus in close association with humans. In areas where Ae. aegypti has been eliminated or suppressed, urban yellow fever has disappeared. Urban yellow fever can be prevented by vaccinating human populations at risk for infection or by suppressing populations of Ae. aegypti mosquitoes so that they no longer perpetuate infection.
Jungle yellow fever can most effectively be prevented by vaccination of human populations at risk for exposure.