The assumption of rational choice helps in understanding how people respond to infectious diseases. People maximize their well-being by choosing levels of prevention and therapy subject to the constraints they face. Objectives and constraints are numerous, necessitating tradeoffs. For example, this approach predicts how people respond to changes in the risk of infection and to the availability of diagnostic tests. The combination of individual rationality with epidemiological models of infection dynamics predicts whether individual choices about infectious disease prevention and therapies produce the best possible social outcomes. If not, individuals' choices generate rationales for government interventions to influence the levels of preventive and therapeutic activities. Optimal policy usually means accepting endemic infection, but at a level lowered by a coordinated package of interventions. Economics combined with epidemiology provides much qualitative guidance on the design of such packages, including immunization programs.