Climate change creates new uncertainties about the spread of diseases such as the Zika virus, dengue fever, malaria, and Lyme disease, which are carried by insects known as vectors, like mosquitoes, ticks, and flies. By altering conditions—local temperatures, rainfall and population movements—that determine the spread of the pathogens, global warming makes the transmission of vector-borne diseases (VBDs) unpredictable and difficult to control. When it comes to VBDs like Zika, climate change is a threat multiplier.