Prompt treatment of infectious diseases plays an important role in infection control. In the face of the increasing incidence of sexually transmitted infection, the ability of genitourinary medicine services to provide appropriate and timely care is reduced. To explore the relationship between capacity and demand for care, we developed and analyzed a mathematical model of gonorrhea transmission, incorporating patient flow through treatment services and heterogeneity in sexual risk behavior. Two equilibrium levels of infection incidence--"high" and "low"--exist for the same parameter values, and which of them occurs depends on starting conditions. At the high-incidence equilibrium, there is a "vicious circle" in which inadequate treatment capacity leads to many untreated infections, generating further high incidence and high demand and thus maintaining the inadequacy of services. A substantial increase in capacity is needed to interrupt this process and enter a "virtuous circle," in which adequate service provision keeps demand low, offering cost savings as well as improvements in health.