The distribution and abundance of Borrelia burgdorferi, including human Lyme disease strains, is a function of its interactions with vertebrate species. We present a mathematical model describing important ecologic interactions affecting the distribution and abundance of B. burgdorferi strains, marked by the allele at the outer surface protein C locus, in Ixodes scapularis ticks, the principal vector. The frequency of each strain in ticks can be explained by the vertebrate species composition, the density of each vertebrate species, the number of ticks that feed on individuals of each species, and the rate at which those ticks acquire different strains. The model results are consistent with empirical data collected in a major Lyme disease focus in New England. An applicable extension of these results would be to predict the proportion of ticks carrying human infectious strains of B. burgdorferi from disease host densities and thus predict the local risk of contracting Lyme disease.