Deer mouse density was calculated by using the Distance program (28) . Mammal species diversity was measured by using the Simpson diversity index (D S ) (29) , which takes into account both richness (number of species) and evenness (number of individuals within each species) and ranges from 0 (least diversity) to 1 (maximal diversity). D S further represents the probability of interspecies encounters (30) . Pairwise comparisons of D S values among parks was conducted by using the Student t test; differences of D S were divided by the square root of their variances (30) . To minimize the possibility of type 2 errors resulting from multiple comparisons, a statistically conservative Bonferroni correction was made (α = 0.05/10 comparisons, or 0.005) (31) . Deer mouse densities were compared pairwise by using the Welch 2-sample t-test. Logistic regression with binomial errors was initially used to assess the association between infection prevalence and deer mouse density and species diversity. However, the resulting models showed such extensive overdispersion that we considered logistic regression to be an unsuitable statistical method for these data (32) . Accordingly, we used nonlinear regression analysis.