When a mosquito-borne disease first arrived in the Western Hemisphere last year, humans were relatively lucky. The disease, which causes crippling joint pain persisting for weeks or even months and for which there is no known therapy or vaccine, hopscotched from the Caribbean islands to eventually land in the United States and the rest of the Americas. But the type of chikungunya creeping across the region then was one that could only readily spread via Aedes aegypti, a mosquito that is uncommon in the United States. Brazil’s summer starts next month, a season of copious rain that will create more ideal breeding grounds for the mosquitoes, which can then go on to bite humans and spread chikungunya. The appearance of the African genotype of chikungunya “is just going to make a bad situation worse,” says Scott Weaver, an expert in human infections and immunity at UTMB.