In Brazil, for generations, yellow fever sat in the Amazon basin, where the Haemagogus and Sabethes mosquitoes thrived, making the disease a concern for those who lived, worked, or traveled deep into the jungle, but only for them; in 2016, it left the Amazon, as more and more mosquitoes fanned out of the rain forest; and by 2017 it had reached areas around the country’s megalopolises, São Paulo and Rio de Janeiro—more than thirty million people, many of them living in shantytowns, facing the arrival of a disease that kills between 3 and 8 percent of those infected.
