Since 1933 certain epidemiologists have held that yellow fever probably is primarily a disease of some of the lower animals, with its normal habitat in tropical and semitropical forested areas; that it only incidentally attacks man; and that it does not frequently become established in a human community. It may be classified as urban, rural, or jungle in type, according to the locality in which it appears and the vectors considered responsible. The urban and rural varieties occur in cities and rural communities where the vector is the Aëdes aegypti (Stegomyia) mosquito and the host is man. Less is known of jungle yellow fever, although it is believed to be transmitted by more than one species of blood-sucking arthropod and to attack wild monkeys and other mammals.
As seen in man the clinical signs and symptoms are apparently the same in the urban, the rural, and the jungle form, and the histopathological preparations of affected organs are indistinguishable in all three.