The agent which causes yellow fever is a mosquito-borne virus of the family Flaviviridae. Occasionally, humans acquire yellow fever in the jungle and then move to a village or urban area where they may be fed upon by domestic species of mosquitos, especially Aedes aegypti, which is also a carrier of the virus. Many major cities of Africa are infested with Aedes aegypti mosquitos, and thousands of deaths could rapidly ensue from the introduction of yellow fever before a vaccination campaign could have any effect.
Leucopenia appears early, possibly together with haemorrhagic symptoms.
Jaundice is moderate early and intensifies later. As the disease progresses there is slowed and weakened pulse, albuminuria and at times anuria. Several hundred to a few thousand cases are reported annually. Rural populations are at greatest risk, with most adult cases occurring among young adult males who enter forests as part of their work.