Abstract: Myxobolus cerebralis, the parasite that causes whirling disease, has been a major contributor to the reduction in trout populations in the Intermountain West, U.S.A. Myxobolus cerebralis has a obligate, two host life cycle: trout produce spores that are infective to the tubificid worm host, Tubifex tubifex, and the worm produces spores that are infective to trout. Evidence collected from both field surveys in watersheds in Montana as well as laboratory experiments suggest that the structure of the worm community plays an important role in the prevalence and severity of disease in fish. Tubifex tubifex is comprised of several genetic lineages that often coexist in stream communities. Our field work has shown lineage I and III (TI and TIII) coexist in Montana streams. Several studies have shown that TIII is a highly susceptible host, more likely to be infected than TI and sometimes producing ten times the number of spores that infect fish than TI. Our field studies using caged trout fry have also shown infection of young fish is directly linked to the density of infected T. tubifex and the most susceptible worm, TIII, dominated the communities in areas of highest whirling disease risk at both within and among watershed spatial scales. Laboratory experiments manipulating communities including the abundances of high, low and unsusceptible hosts have revealed complex interactions within the worm community that influence the production of number of spores that are infective to fish. For example, the presence of a worm taxon that cannot transmit the parasite did not decrease the prevalence of infection in highly susceptible TIII; however, infection prevalence in TIII was low when it coexisted with a low susceptibility lineage, but the result was not statistically significant. Thus, the complex ecological interactions within worm communities that determine parasite success are the focus of our current research.