They are transmitted by the bite of a mosquito. The two major agents that cause systemic disease are the yellow fever and the dengue viruses. They are prevalent in the Caribbean basin, South America, and Africa. There are many other important flaviviruses including in the US, the St. Louis encephalitis virus. Although most of the flaviviruses are neurotropic, these two viruses are primarily visceromorphic and pantamorphic meaning that they cause hemorrhagic and systemic disease.
Yellow fever presents with the abrupt onset of fever and chills.
It is associated with hepatic necrosis and dysfunction with bleeding, shock, renal dysfunction, lymphoid necrosis, and myocarditis. Pathologically there are councilman bodies in the liver with hypertrophy and necrosis of kupffer cells and hepatocytes. The disease of YF is due to direct viral injury to the liver and lymph nodes not immunopathologic mechanisms. Recovery from YF is associated with the development of life-long immunity. Thanks to the work of the US Yellow Fever Study Group, there is an excellent vaccine for YF derived from one of the first strains of YF isolated from a patient.