preferred alternative use, assuming the illness had not arisen. A measure of the economic burden of STDs thus indicates the quantity of resources that would be available for other purposes if they were not used to address STDs. The actual source of funds may be the government, individuals, or other parties such as insurers. In some cases, resources may be used with no accompanying transfer of money.

The costs associated with STDs and other illnesses include "direct" costs, both medical and nonmedical, for services and materials. Major categories of direct costs incurred because of illness include the costs of health care workers' services (those of physicians, nurses, technicians, and others), costs associated with hospital admissions, costs of pharmaceuticals, and costs of medical equipment. The costs of transportation, home care, special schooling, and other such resources are also direct costs.

Productivity costs are also incurred by illness. These costs, often termed "indirect" costs, reflect the opportunity cost of the productive time the affected individual (and society) lose because of illness. Productivity costs include both time spent sick, when an individual is unable to engage in the activities he or she otherwise would, and premature mortality. The time an individual invests in treatment-traveling to obtain care, waiting in the clinic waiting room, filling a prescription, undergoing a procedure-could be categorized either as a direct cost or a productivity cost. Regardless of how it is categorized, it entails an opportunity cost.

The economic burden of a disease can be demonstrated by calculating either its prevalent costs (the annual costs of cases prevalent during a given year) or the lifetime costs of individual cases (incident costs). This review focuses on the former measure of economic burden, often termed the "cost-of-illness." This approach seeks to capture the costs associated with the cross-section of existing cases-including current costs for cases that occurred previously-but does not reflect future costs associated with a disease. Prevalent costs provide an estimate of economic burden that is useful for comparisons in an annual context, such as for federal, state, or institutional budgets. Incident costs, in contrast, demonstrate the full impact of an illness over time on a cost-per-case basis. They include the present value of future costs associated with the cases occurring during a given year.

For some illnesses, the distinction between prevalent and incident costs is of minimal importance, as the primary costs of the disease relate to the acute infection and occur during the first year of the disease. For most illnesses, however, costs occur over a longer period of time. This is clearly true for debilitating congenital illnesses, for which costs of care may extend over a lifetime. It is also true for STDs that have long-term consequences, such as pelvic inflammatory disease (PID) and infertility. For this reason, the prevalent costs of STDs cannot be assumed to reflect the full costs per case of these diseases.

This effort focuses on the collection and adjustment of existing estimates. It

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