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
Yellow fever and vaccination is uncommon, early diagnosis of cases and immunization of large parts of the population are important to prevent outbreaks. Once infected, management is symptomatic with no specific measures effective against the virus. Death occurs in up to half of those who get severe disease. In 2013, yellow fever resulted in about 127,000 severe infections and 45,000 deaths, with nearly 90% of these occurring in African nations. Nearly a billion people live in an area of the world where the disease is common. It is common in tropical areas of the continents of South America and Africa, but not