Last month, a California resident was infected with dengue fever after a bite from a local mosquito. This case made the news because it was the first, but it may not be last. Dengue and another mosquito-transmitted disease, malaria, are on the rise around the world. Several factors are behind this trend. Climate change allows mosquitoes to live in broader geographic regions. A city-dwelling mosquito species has been transported across continents and established itself in dense urban areas. And, importantly, mosquitoes have changed.