Ecological and environmental factors can influence infectious disease transmission via host physiology and behavior. Using the North American deermouse (Peromyscus maniculatus), the primary host for the directly-transmitted Sin Nombre hantavirus (SNV), we investigated how heterospecific competitors and seasonality affect host susceptibility to infection and intraspecific contact rates, key mechanisms of transmission. In grasslands of western Montana, deermice compete with voles (Microtus spp.) and shrews (Sorex spp.). We hypothesized that dominant voles, and less so shrews, will induce chronic stress, suppress immunity, and may change deermouse contact rates, and during spring/summer, deermice may experience chronic stress, suppressed immunity, and higher contact rates. We trapped small mammals, collected feces and blood from deermice, and evaluated them for scar numbers and body condition scores (BCSs). We evaluated stress physiology with fecal corticosterone metabolites (FCMs), neutrophil/lymphocyte (N/L) ratios and BCSs, immunity with total white blood cell (WBC) counts, and contact rates with scar numbers. We found that shrew density negatively correlated with stress response FCMs, and although complex interactions existed, shrew and vole densities negatively correlated with BCSs but differentially with scar numbers. N/L ratios were higher in spring/summer whereas WBC counts were lower in summer, indicative of chronic stress and immunosuppression, respectively. Our results suggest that heterospecific competitors could differentially influence disease transmission, and that chronic stress, immunosuppression, and higher contact rates may help explain higher SNV transmission previously reported in Montana during spring/summer .