Yellow fever—a mosquito-borne virus that can cause fever, chills, muscle aches, headaches, and (in serious cases) bleeding, organ failure, and death—has long been endemic to Brazil. First brought to the Americas from Africa in the 1600s during the slave trade, yellow fever used to kill hundreds of thousands of people annually throughout the region. That changed in the early 1900s with aggressive regional Aedes aegypti mosquito elimination programs in cities (Aedes aegypti being the primary mosquito to transmit yellow fever in urban settings), the development of an effective yellow fever vaccine, and the implementation of large-scale urban immunization campaigns. These combined efforts caused the number of human fatalities in Latin America to drop exponentially and largely confined the virus to monkey populations living deep in the Amazon.
— June 23, 2017