Understanding how animals use sensory information in the environment requires knowing the structure of the relevant stimulus space in nature. For instance, vision researchers must examine how often specific colors and patterns occur in relevant stimuli. Characterizing olfactory stimuli is especially difficult. A given stimulus could comprise hundreds of individual compounds, each of which might be considered its own dimension in odor space. Trapping and analyzing these complex blends is technically challenging. Thus, stimulus distribution in odor space (the frequency with which compounds and their combinations occur in nature) remains largely obscure. Here, we undertake the first large-scale survey of an odor space relevant to blood-feeding insects: vertebrate body odors. We confirm previous evidence that these odors are mostly composed of ubiquitous compounds; measuring any one compound provides minimal information about stimulus identity. Thus, blood feeders’ sensory systems must be tuned not to single, host-specific compounds but to distinctive compound ratios in an odor of interest. We describe the major axes that separate vertebrate odor from other environmental odors and discuss candidate compounds that blood-feeding insects may use to target hosts. We also describe how vertebrate odor varies across species, including consistent differences between human and non-human odor that result from unique aspects of human skin biochemistry. Prior evidence suggests human-specialist mosquitoes indeed exploit these cues. By analyzing the structure of vertebrate odor space, our work generates predictions about how a sensory system should carve out a section of odor space to focus on relevant stimuli. These results may also inform the development of novel baits or repellents that can be used to manipulate the behavior of blood-feeding insects.