Small, isolated populations often experience reduced genetic diversity; the reason why some do and some do not exhibit increased disease susceptibility has been much debated. Comprehension of these patterns is vital to informing management programs of wild and captive populations. In order to add to this understanding, I assessed the immune strength of inbred deer mice in Lake Michigan directly through antigen (PHA) challenge and indirectly through measurement of parasite loads and leukocyte counts. Populations of deer mice on three islands, with areas of 2300, 3600 and 4600 acres respectively, were trapped during July 2009. Immune response, assessed as a 6 hour reaction to PHA injection, was measured on each mouse; blood and fecal samples were collected as well. Immunocompetence was assessed through total leukocyte counts, neutrophil: lymphocyte ratios, fecal oocyte counts and hematozoan prevalence. Genetic analysis was conducted with 5 microsatellite markers. Observed heterozygosity and heterozygosity by locus were used as measures of homozygosity and inbreeding, respectively. I investigated the relationship between diversity, infection and immune status through a series of t-tests, ANOVAs and linear models. I found that parasite species richness and prevalence of endoparasitic but not hematozoan infection, generally decreased with island size. Smaller islands had higher levels of homozygosity and inbreeding. More homozygous individuals tended to be infected in all populations but there was no association with inbreeding. Relationships between stress level, inbreeding and intensity of infection were specific to each island population. Inbreeding was positively correlated with immune response but genotypes and environmental pressures drove how that relationship was exhibited. The results of my study emphasize the importance for management programs to understand the full ecological context of a population in order to forecast how introducing novel experiences will impact them.