The spread of many infectious diseases occurs in a diverse population, so that it is desirable to include heterogeneity in the formulation of the epidemiological model in order to improve its predictive and explanatory power and its applicability. Often the heterogeneous population is divided into subpopulations or groups, each of which is homogeneous in the sense that the group members have similar characteristics. This division into groups can be based not only on disease-related factors such as mode of transmission, latent period, infectious period, genetic susceptibility or resistance, and amount of vaccination or chemotherapy, but also on social, cultural, economic, demographic or geographic factors. For example, the mixing behavior may depend on the age of the individuals. If any of the epidemiological characteristics are gender dependent, then groups of men and women may be necessary.

The transmission of sexually transmitted diseases (STDs) often occurs in a very heterogeneous population. People with many different sexual partners have many more opportunities to be infected and to infect others than people who have fewer partners. Thus for STDs it is often necessary to divide the population on the basis of the amount of sexual activity. Frequently, the epidemiological characteristics of STDs are different for men and women. For example, the probability of transmission per partner of gonorrhea from male to female is greater than that from female to male. Moreover, the fraction of women with gonorrhea who are asymptomatic is larger than the fraction for men (Hethcote and Yorke 1984).