Rather than confining the categories health and sickness to a biomedical conception of the biological organism, there is growing recognition of epistemological and ontological multiplicity in the realm of diagnosis and, indeed, in the very realm of disease itself. In short, the empirical manifestations of health and illness as well as the processes thought to cause them are now understood to assume a much wider variety of both biological and other forms. This essay considers the underlying epistemological and ontological opportunities and challenges of taking what we are calling this diffusion of diagnosis seriously. By diffusion we mean the movement from a concentrated understanding of diagnostic authority as confined to scientific biomedicine to a less concentrated appreciation of the diverse approaches to diagnosis throughout the world. We consider the extent to which, and the manner in which, we as sociologists of diagnosis might not only critique these various processes but perhaps also take them seriously in an ethnographic sense as locally produced, evaluated and legitimated forms of health care.