Little is known about how the risk of disease varies across species and its consequences for host defenses, including the immune system. I obtained mean values of basal white blood cells (WBC) from 100 species of primates to quantify disease risk, based on the assumption that higher baseline WBC counts will be found in species that experience greater risk of acquiring infectious disease. These data were used to investigate four hypotheses: disease risk is expected to increase with (1) group size and population density; (2) greater contact with soil-borne pathogens during terrestrial locomotion; (3) a slow life history; and (4) increased mating promiscuity. After controlling for phylogeny, WBC counts increased with female mating promiscuity, as reflected in discrete categories of partner number, relative testes mass, and estrous duration. By comparison, the social, ecological, and life-history hypotheses were unsupported in comparative tests. In terms of confounding variables, some WBC types were associated with body mass or activity period, but these variables could not account for the association with mating promiscuity. Several factors may explain why hypotheses involving social, ecological, and life-history factors went unsupported in these tests, including the role of behavioral counterstrategies to disease, restrictions on female choice of mating partners, and the effect of transmission mode on parasite strategies and host defenses.