Other ecological factors could affect the number of intraspecific deer mouse (Peromyscus maniculatus) encounters, including increased density of deer mice and vegetative factors that lead to variation in population numbers (e.g., available cover and forage) ( Table 1 ). Some studies have found high SNV prevalence in host populations when deer mice densities were high (18, 19) . However, although the concept of density-dependent transmission is not unique to hantaviruses, its applicability to the deer mouse-SNV system has been elusive. SNV prevalence also has been shown to vary with habitat characteristics and quality (15, 18, 19) , although interpretation of this variation has been difficult because SNV prevalence varies as much within as among habitat types (20) .