Yellow fever seems to play a role in single, sudden breakouts of the disease. Three epidemiologically different infectious cycles occur, in which the virus is transmitted from mosquitoes to humans or other primates. In the "urban cycle", only the yellow fever mosquito "A. aegypti" is involved. It is well adapted to urban areas, and can also transmit other diseases, including Zika fever, dengue fever, and chikungunya. The urban cycle is responsible for the major outbreaks of yellow fever that occur in Africa. Except for an outbreak in Bolivia in 1999, this urban cycle no longer exists in South America. Besides the urban
1793 Philadelphia yellow fever epidemic vector of the 1793 plague in Philadelphia (and other Atlantic ports) was the British merchant ship "Hankey", which had fled the West African colony of Bolama (an island off West Africa, present day Guinea-Bissau) the previous November, trailing yellow fever at every port of call in the Caribbean and eastern Atlantic seaboard. See: "The Ship of Death: The Voyage That Changed the Atlantic World". After two weeks and an increasing number of fever cases, Dr. Benjamin Rush, a doctor's apprentice during the city's 1762 yellow fever epidemic, saw the pattern; he recognized that yellow fever had returned. Rush alerted his