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. The recent marriage of economics and epidemiology, though long delayed, promises to be fruitful. Public health means public policy, never more so than with respect to infectious diseases. Economics has much to say about public policy, and epidemiology has much to say about the dynamics of infectious diseases. This article reports on the first progeny of this marriage-vigorous and precocious in what they say about public policy, but with potential still to fulfill. Infectious disease has two modes of transmission: from person to person, as with HIV/AIDS, or by vectors, such as mosquitoes, as with malaria or yellow fever. Both modes are strongly influenced by how people choose to act both individually and collectively through their governments. People make choices about prevention and therapy from among the options available to them. An important influence on these