A little-known pathogen named the Oropouche virus is on the move in South America, alarming scientists and public health experts. Brazil has reported 5530 cases so far this year, compared with 836 in all of 2023. Bolivia, Colombia, and Peru have seen upticks as well. Although the virus has traditionally been endemic in the Amazon Basin, it is now sickening people far from the rainforest. In May, Cuba reported its first cases.

Most cases of Oropouche fever are mild, with symptoms such as headache, body pains, nausea, and rash—but the virus can also cause brain inflammation and neurological problems, including vertigo and lethargy. And even a mild epidemic could overwhelm the continent’s health systems.

“What worries us most is the expansion of a disease that was practically restricted to the Amazon, which has a very low-density population, to areas with greater population density.”