We incorporate basic genetics into an SIS endemic model of a disease that is spread primarily by individual–individual contact after it is introduced into the population. We illustrate that if a homozygote is immune to the disease or is resistant to the effects of the disease, the corresponding allele goes to fixation. On the other hand, if the heterozygote is immune to the disease or is resistant to the effects of the disease, polymorphism usually occurs. However, under some conditions, polymorphism may not occur.