Infection of hosts by multiple parasite species is the norm rather than the exception in most natural populations, yet studies of parasite dynamics largely focus on single parasites interacting with single hosts. Fundamental principles of immunology suggest that co-infection of hosts by microparasites (bacteria, viruses, protozoa) and macroparasites (helminths) should have important effects on disease dynamics. For example, exposure to macroparasites may increase host susceptibility to and progression of important microparasitic diseases such as AIDS, tuberculosis and malaria in humans. The central goal of this study is to investigate the consequences of microparasite-macroparasite interactions for patterns of disease at three distinct levels of biological organization: individuals, populations and species. To achieve this goal, African buffalo (Syncerus caffer) co-infected with bovine tuberculosis (BTB, Mycobacterium bovis) and gastrointestinal nematodes will be used as a model system to investigate individual-level patterns of infection and parasite population dynamics. Specifically, a combination of field and captive studies and mathematical modeling will be used to assess patterns of infection and immunity in these free-ranging animals, understand the effects of nematode treatment on BTB transmission, and examine the reciprocal effects of BTB on nematodes. Scaling up from this system, a comprehensive database of parasites and pathogens infecting wild mammals and humans will then be used to investigate the effect of helminths on the distribution of microparasitic diseases across populations and species. In combination, these analyses will reveal how within-host immunological interactions between micoparasites and macroparasites shape patterns of disease in natural host populations and among species. Broader impacts of this study include training of multiple graduate students, undergraduate students and paraprofessionals; application of study results to the management of an ecologically and economically important wildlife disease; and active collaboration with the public health community to understand the utility of using helminth control as a tool for combating microparasitic diseases in humans.