The historical study of Western medicine in nineteenth-century Siam has emphasized the dichotomy between Western medicine and traditional Thai medical practice. The former is often represented as a monolith, and the epistemological transformation of Western medicine during the nineteenth century is glossed over without sufficient attention. Pasteurian medicine, especially the idea of germs, was introduced to Siam by the American missionary Dan Beach Bradley. Its introduction spurred a process of negotiation with both pre-Pasteurian Western and traditional Thai medicine. In its pre-Pasteurian and Pasteurian variants, Western medicine was constituted as a new medical practice and disciplinary regime in Siam. As a discursive instrument of state hegemony, the ideas, structures, policies, and institutions of Western medicine furthered the understanding and management of virulent epidemics, the institution of the sanitary system, the shaping of new concepts of population and a healthy workforce, and not least, the framing of a medicalizing project to police people's bodies pursued by the Thai state in the 1930s.