Thinking of AIDS as an 'emerging disease' inevitably raises questions of comparison. In the United States, we see three main phases in understanding AIDS, with each having very different implications for health and social policy. In the first, AIDS was conceived of as an epidemic disease, a 'gay plague', by analogy to the sudden, devastating epidemics of the past. In the second, it was normalized as a chronic disease, similar in many ways to diseases such as cancer. In the third, (...) we outline a new understanding of AIDS as a slow-moving, long-lasting pandemic, a chronic infectious ailment manifested through myriad specific HIV-related diseases. This new paradigm emphasizes, like the plague model, the etiology, transmission, and prevention of disease; like the chronic disease model, it is concerned with the clinical management of protracted illness. We do criticize, however, both the infectious and chronic disease models for their individualistic conceptions of disease and their narrow strategies for disease prevention. We further suggest that the traditional distinction between, and approaches to, infectious and chronic diseases are problematic and need to be rethought for AIDS and other diseases. (shrink)