We are exploring the molecules and neural circuits that cause mosquitoes to seek out and bite humans. The mosquitoes that transmit dengue fever show an innate preference for biting humans over other animals. But not all members of the species prefer humans: A forest-dwelling African form favors animals. As a postdoctoral fellow, I brought human and animal-loving mosquitoes from nature into the laboratory, and-- by comparing their genes and gene products-- discovered an odorant receptor in mosquito antennae that helps disease-transmitting mosquitoes home in on a particular chemical present in human odor. We are now characterizing additional odorant receptors and other genes that underlie the evolution of preference for humans. We will also image the brains of live mosquitoes to look for changes in the activity or anatomy of the brain regions that process odors. Understanding the biological basis of host preference could inform efforts to curb the spread of dengue and other mosquito-borne diseases, in addition to adding to our basic understanding of sensory processing and evolution.