Participant observation fieldwork among street-level heroin injectors in San Francisco demonstrates the need for contextualized understandings of how power relations structure individual behavior in the transmission of HIV. Problematizing macro/micro dichotomies, we explore how externally-imposed power constraints are expressed in everyday practices constituting differential HIV infection rates within distinct population groups. The pragmatics of income-generating strategies and the symbolic hierarchies of respect and identity shape risky behavior. The political economy and symbolic representations of race, class, gender, sexuality, and geography organize chronic social suffering and distort research data. Traditional paradigms of applied public health elide power relations and overemphasize individual behavior. Ignoring the centrality of power prevents a full understanding of the who, why, how, and where of HIV infection. (Authors)