 Health and disease, like many other concepts, are neither purely scientific nor exclusively a part of common sense. They have a home in both scientific theories and everyday thought. That raises a problem for any philosophical account: suppose we try to say what health and disease really amount to, from which it follows that the scientific concept should fit the facts about world. If the picture we end up with deviates too far from folk thought, should we worry? You might think that everyday language puts constraints on a concept of health that need to respected, and that if we move too far from ordinary usage we have stopped talking about health and started talking about something else. Furthermore, it is not really possible to argue that scientific and vernacular uses of the concepts are fully independent, since the development of science influences everyday thought, and many scientific concepts begin in pre-scientific contexts and carry the marks of those origins deep into their careers.