many partners ("assortative mixing"), infection spreads rapidly at first; and it spreads fastest within "dense" sexual networks with many sexual links over a short period of time (Morris, 1993). Thus, interventions should have the greatest impact if they reach, and are effective among, individuals who have many partners and in "dense" networks with "assortative mixing."

Primary Versus Secondary Prevention

For many infectious diseases, vaccines are a major method of prevention. It is important to recognize that early detection and curative treatment of individuals with communicable diseases provide not only secondary prevention at the individual level, but also primary prevention at the population level by preventing further transmission. Reduction in the duration of infectivity, particularly among those most likely to transmit the infection to others, lowers the reproductive rate of infection (R0). Thus, public health efforts to prevent the spread of communicable diseases need to include not only immunization programs but also early detection and curative treatment of communicable diseases, especially those for which vaccines are not available. For these diseases, behavioral interventions are also important.

Behavioral Versus Biomedical Approaches

Historically, STD programs have been based on a biomedical model that focused on the treatment of infected individuals and the development of biological and biomechanical interventions such as drugs, diagnostic tests, and barrier methods. Services have centered on the medical screening and treatment of individuals, coupled in some cases with partner notification. The system for delivering services typically has been composed of health professionals practicing in fixed clinical settings. Traditionally, STD prevention activities have involved episodic therapy driven by symptoms of disease and have provided limited clinical counseling or education to promote behavior change. In recent years, the approach to STD prevention has begun to change as a result of critiques by both affected communities and social and behavioral scientists (Fee and Krieger, 1993). Both biomedical and behavioral health disciplines have made important contributions to the knowledge base for STD prevention (Sparling and Aral, 1991). Because both behavioral and biomedical approaches to STD prevention are necessary, distinguishing between them is unimportant. Federal agencies recently have recognized the need to incorporate both behavioral and biomedical approaches in a more holistic approach.

Wasserheit (1994) examined six changes in patterns of STDs and described how physical and social environmental changes drive these disease patterns. She called for the development of STD prevention programs based on "an appreciation of the role of risk behaviors and macroenvironmental forces" using companion



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