It affects principally, monkeys and other vertebrates in tropical America and Africa, and is transmitted to man by certain culicine mosquitoes. In general, death occurs between the fifth and tenth day of illness. The case fatality rate may reach 80 per cent in severe cases.
Survivors exhibit long-lasting immunity.
The endemic zone, defined by the presence of neutralizing antibodies, includes a total of 29 countries where epidemics occur at irregular intervals. S. Most cases are reported from Bolivia, Brazil, Colombia and Peru. Paradoxically, there is no evidence that yellow fever ever has been present in Asia, where the closely related disease, Dengue is endemic. However, whether Dengue immunity would result in cross-protection against yellow fever in man is not certain, especially since yellow fever immunization and control does not protect against Dengue. Under natural conditions, the virus is pantropic built after continued culture in tissues, as in chick embryo, it loses all its pathogenic properties but retains its antigenicity.