Theoretical consideration of the epidemiology of sexually transmitted diseases (STD) suggests that the key determinants of transmission dynamics are the duration of infectiousness and the extent to which subgroups in the population interact sexually. We used two empirical correlates to represent these concepts: (1) the force of infectivity, calculated by summing all the days of potential infectivity (the time between last sexual exposure and treatment of the contact) generated by a given case, and then summing the days for all cases within a given subgroup; (2) self-selection, representing the observed probability that members of a given subgroup select sexual partners from within their own group. Using data gathered in Colorado Springs, Colorado, we estimated that a single group i.e., black, male, heterosexual, military personnel residing in the core areas, generated 27% of the force of infectivity. Subgroups that select greater than 50% of their sexual partners from outside their sociodemographic boundaries generated a rate for the force of infectivity that was 4.5 times higher than the rate for self-selectors. These findings confirm the core group theory and suggest strategies for control of STD.