Yellow fever virus is a member of the genus Flavivirus, a single-stranded RNA virus found in tropical areas of South America and Africa. It is transmitted by the bite of infected Aedes, Haemagogus, or Sabethes species mosquitoes. The reservoir host is a human or nonhuman primate.
There is a jungle cycle of transmission of the virus between monkeys and mosquitoes that live in the canopy of the forest. Humans can become infected when they enter the jungle. There is also a cycle in the savannah of Africa between mosquitoes and people who live near the jungle. The urban cycle involves Aedes aegypti and humans. A person infected in the jungle or savannah is bitten by the Ae. aegypti mosquito and then the mosquito bites other humans in urban areas.