Summer is here. The heat is rising, the rains are getting heavier, and across the world, health officials are watching something that used to be someone else’s problem creep into their own backyards. Dengue fever was once locked inside tropical and subtropical belts—the kind of disease you only worried about if you were traveling to Southeast Asia or Latin America. That is no longer true. Over the past few years, this mosquito-borne disease has pushed further north than most people expected. And the latest evidence suggests it is picking up speed. What is dengue fever, really Dengue is caused by the dengue virus, which spreads to humans through the bite…