Classifying patients to diagnose and treat disease, ensure access to medical care, adhere to standards of quality, contain costs, and fulfill contractual obligations is critical to the delivery of healthcare. While classification is a fundamental standardizing process in healthcare, as a social process it is the product of negotiations, organizational processes, and moral conflict often hidden in bureaucratic and professional modus operandi. By comparing the early twentieth century case of syphilis to the contemporary case of HIV/AIDS, the dissertation shows how symptomological, etiological, and financial classification have been developed and deployed in the context of a transforming twentieth century American medical system; how those deployments have been sustained, modified, and/or undermined over time; and, ultimately, how classifications influence the social construction of disease through the creation of social problems and their solutions. The layering of classifications over time can facilitate the unintended and obscured persistence of categories and criteria, impacting the day-to-day practice of sorting patients, treatments, and experts. The aim of the dissertation is to study the social process of classification in its everyday operation in the interplay between discourse and practice to understand what healthcare providers actually do with the classifications they are handed, including the adaptation of classifications to local situations that, ultimately shape the way syphilis and HIV/AIDS are integrated into medical practice itself. As a result of looking at classification this way, I show how symptoms, diseases, patients, treatments, and medical care providers are dynamic objects of classification that contribute to collective definitions of disease and influence how medicine and public health organize activity in changing technological, administrative, and moral contexts.