The germ theory of infectious disease, which developed in the late nineteenth and early twentieth centuries, is often considered a pivotal breakthrough in modern science, medicine, biology, and public health. The germ theory provided a new way to study disease in laboratory, clinical, and community settings, and a new rationale for public health intervention. This article explores two important facets of the germ theory; how the physical techniques and methods of studying germs in laboratories were taught to the first generation of doctors, and how the germ theory was communicated to diverse publics in clinical and community settings. Drawing on the concept of transnational science, I argue that late nineteenth and early twentieth debates around the laboratory practices of bacteriology and the public reception of the germ theory help us to understand the deeper ways that biomedical scientific knowledge is created, constrained, and communicated.