The articulation and acceptance of specific disease entities constitute one of the most important intellectual and cultural events of the past two centuries. This notion is central to how we organize health care delivery, think about ourselves, debate and formulate social policy, and define and manage deviance. Diagnosis is indispensable to linking specific disease concepts with doctor and patient and the social and economic institutions shaping such clinical interactions. Disease is a social entity, not an array of ideal types. The history of medicine is partly the story of how disease entities have become social entities, accumulating the flesh of diagnostic and therapeutic practice, social expectation, and bureaucratic reification. Despite criticism of reductionist medicine in the West and less focus on disease entities and mechanisms, our social response still depends on this concept of sickness. But this concept can no longer remain invisible if we are to understand contemporary medicine as both a social and a technological system.