Host genetic diversity is thought to reduce the likelihood that disease will spread in natural populations. In this study, I present an epidemiological model for the intrinsic rate of spread (R0) for an infectious disease. The results show that the average value for R0 (R0) is inversely related to the number of host genotypes in the population (G), assuming that each host genotype is susceptible to a different parasite genotype. Specifically, for large host populations, R0 is equal to B/G, where B is the number of infectious propagules produced by each infection that contact a different host. The results also suggest that virulent, single-strain infections, which initially spread in genetically diverse host populations, would quickly die out when the parasite depresses the frequency of susceptible hosts below 1/B. These results are consistent with empirical studies showing that genetically diverse host populations suffer less from pathogens and parasites.