Density of hosts and density-dependent pathogens are highest at the core of species ranges

Infectious diseases are among Earth's most powerful ecological and evolutionary forces, capable of restructuring populations, reshaping entire ecosystems, and destabilizing human societies, as starkly demonstrated by the COVID-19 pandemic. However, the spatial patterns underlying pathogen density remain surprisingly underexplored, and thus it is unclear where risk of spillover to wildlife and humans is greatest. Although controversial1–4, the center-periphery hypothesis predicts that species densities peak at range centers and decline toward edges, yet its application to disease ecology remains largely untested. Here, we first help resolve when and why center–periphery patterns emerge by providing support for the hypothesis using >200,000 standardized monitoring records of 572 fish species and 408 macroinvertebrate genera. Next, using a completely independent global dataset of >4300 parasite prevalence records across >1200 host-pathogen combinations, we reveal that density-dependent pathogens mirror the host density patterns, declining toward range peripheries across amphibians, reptiles, insects, fish, and mammals. In contrast, frequency-dependent pathogens—including vector-borne and sexually transmitted diseases—remain constant across bird ranges or increase toward range edges in mammals. These patterns, robust across taxa and scales, imply that range margins may serve as host refugia from density-dependent pathogens but hotspots for some frequency-dependent diseases. By integrating biogeographic theory with disease ecology, our findings demonstrate that range-wide variation in host density and transmission mode predict broad patterns of pathogen distribution, offering practical guidance for prioritizing disease surveillance and management across species’ ranges.