Parasite infections of individual hosts are determined largely by their exposure, resistance and susceptibility to parasites. Heterogeneity in patterns of infections between individual hosts reflects the outcome of a variety of host-parasite interactions, which will be influenced by factors such as host physiology, behaviour, and habitat, as well as parasite-related factors. Establishing patterns of parasite infection among individual hosts, and identifying the complex interactions that create them, is of interest not only to epidemiologists, but also has relevance within the wider field of ecology. Using a variety of statistical and experimental techniques, this study examined particular patterns and associated processes acting to create heterogeneities in parasite infection in individual red grouse (Lagopus lagopus scoticus), a species of ecological and socio-economic importance. This study began by calibrating the use of faecal egg counts to estimate intensities of infection of the gastrointestinal nematode Trichostrongylus tenuis, an important parasite of red grouse. Analyses of patterns of infection within and between breeding pairs and broods of chicks showed that there was no relationship between infection intensities of paired males and females or between genetically similar individuals. This suggested that both exposure and host resistance may be important in determining an individual’s parasite infection. A cross-fostering experiment was attempted to tease apart the relative contribution of exposure and resistance, but was not successful due to extraneous events. Within breeding pairs, red grouse were found to exhibit positive assortative mating with respect to body condition and, in both sexes, bigger ornamental combs were associated with better body condition, suggesting that comb size might function as a signal of individual quality. Comb size is testosterone dependent and in males, further experimental work showed that experimentally elevated testosterone in male red grouse imposed a cost through reduced immunocompetence and higher intensities of T. tenuis. These interactions have important implications for our understanding of sexual selection and cyclic population dynamics.