For decades, control of yellow fever was a Brazilian success story. The country hadn’t seen a single outbreak of the devastating disease since 1942, thanks primarily to widespread vaccination as well as mosquito control efforts. Then, in 2017, the disease made a dramatic comeback. Cases spread through several cities, infecting more than 2000 people in just 2 years. More than 700 died. Now, researchers have come up with a counterintuitive explanation for the sudden spread of the virus. Altho