Competitive interactions among and between species can drive the emergence and persistence of disease. Hosts often encounter pathogens while feeding, but interactions between hosts for food resources and their associated pathogens have only recently been integrated into studies of disease transmission. Oyster reefs, their commensal filter-feeding fouling communities, and Dermo disease illustrate this central connection between foraging and transmission. Using a series of mesocosm experiments that varied the density of oysters, the dose of the agent of Dermo disease (Perkinsus marinus), and the presence of the commensal, filter-feeding tunicate Molgula manhattensis, we advanced hypotheses of Dermo transmission based on broadly applicable components of resource competition and feeding biology. We found that the presence of commensal tunicates, which were incompetent hosts for Perkinsus marinus, diluted seawater densities of this pathogen through the consumption of suspended particles, and resulted in lower prevalence and intensity of Dermo disease in oysters. We integrated these results in a compartmental disease dynamic model including oysters and M. manhattensis as focal and nonfocal hosts for Perkinsus marinus. The solution to the basic reproduction number R0 from the steady state of the model system demonstrated that increasing the density and filtration rate of nonfocal hosts limited disease risk in the focal host population. Ecological interactions between oysters, their commensal filter-feeding fouling communities, and suspended pathogens in the water column are critical to understanding patterns of disease in oysters.