This article reconstructs the process of defining influenza as an infectious disease in the contexts of British medicine between 1890 and 1914. It shows how professional agreement on its nature and identity involved aligning different forms of knowledge produced in the field (public health), in the clinic (metropolitan hospitals) and in the laboratory (bacteriology). Two factors were crucial to this process: increasing trust in bacteriology and the organisation of large-scale collective investigations into influenza by Britain’s central public authority, the Medical Department of the Local Government Board. These investigations integrated epidemiological, clinical and bacteriological evidence into a new definition of influenza as a specific infection, in which a germ – Bacillus influenzae – was determined as playing a necessary but not sufficient role in its aetiology, transmission and pathogenesis. In defining ‘modern influenza’, bacteriological concepts and techniques were adapted to and selectively incorporated into existing clinical, pathological and epidemiological approaches. Mutual alignment thus was crucial to its construction and, more generally, to shaping developing relationships between laboratory, clinical and public health medicine in turn-of-the-century Britain. While these relationships were marked by tension and conflict, they were also characterised by important patterns of convergence, in which the problems, interests and practices of public health professionals, clinicians and laboratory pathologists were made increasingly commensurable. Rather than retrospectively judge the late nineteenth-century definition of influenza as being based on the wrong microbe, this article argues for the need to examine how it was established through a particular alignment of medical knowledge, which then underpinned medical approaches to the disease up to and beyond the devastating 1918–19 pandemic.