Why do some mosquitoes find us irresistible, preferring to bite human hosts and spread disease with those bites, while others would rather bite another animal? To answer that question, Princeton EEB researchers Lindy McBride and Noah Rose, in collaboration with a large international network of scientists, spent three years driving around sub-Saharan Africa collecting the eggs of Aedes aegypti mosquitoes, which are responsible for Zika, yellow fever, chikungunya, and dengue. They brought the eggs back to the lab, hatched them, and tested their attraction to human odor. Preference for human hosts was linked to two ecological factors: living in dense, urban environments and living in places with long, hot dry seasons, where aquatic mosquito larvae depend on human water storage to survive. Sequencing 375 individual genomes from across the continent showed that this preference likely arose once in seasonal environments, before spreading to cities both inside Africa and across the global tropics. Their models suggest that rapid urban growth could drive the evolution of increased preference for human hosts in cities across sub-Saharan Africa in the next thirty years.