Abstract: Many emerging diseases are zoonotic, but often the dynamics of the disease in wild populations are not well understood. This study aimed to identify the prevalence of Hantavirus and intestinal parasite infection in sympatric Peromyscus maniculatus and P. leucopus populations at the Mountain Lake Biological Station in Giles County, Virginia. Population demographics and immune response were analyzed to determine if there is an immune mediated response to infection with either Hantavirus or intestinal parasites, in addition to concurrent infection with both pathogens. Most likely due to low population densities, antibodies for Hantavirus were not found in the population, but approximately 80% of the mice were infected with intestinal parasites. These parasites were found to marginally increase white blood cell counts, but were shown to have a significant negative effect on mouse body condition (P < 0.0105). The effect of infection on lower body condition and elevated immune response could be imposing a fitness effect on the mice, so that mice with intestinal parasites have lower survival or reproductive rates.