Yellow fever cycle, both in Africa and South America, a sylvatic cycle (forest or jungle cycle) is present, where "Aedes africanus" (in Africa) or mosquitoes of the genus "Haemagogus" and "Sabethes" (in South America) serve as vectors. In the jungle, the mosquitoes infect mainly nonhuman primates; the disease is mostly asymptomatic in African primates. In South America, the sylvatic cycle is currently the only way humans can become infected, which explains the low incidence of yellow fever cases on the continent. People who become infected in the jungle can carry the virus to urban areas, where "A. aegypti" acts as a vector.
Yellow fever vaccine Foundation to improve the vaccine from the "Asibi" strain, discovered that a favorable chance mutation in the attenuated virus had produced a highly efficacious strain that was named 17D. After field trials in Brazil, over one million people were vaccinated by 1939, without severe complications. For his work on the yellow fever vaccine, Theiler received the 1951 Nobel Prize in Physiology or Medicine. Only the 17D vaccine remains in use today. The outbreak of yellow fever in Angola in January 2016 has raised concerns about whether the global supply of vaccine is adequate to meet the need during a large