This paper analyzes how the local physicians in Bogota understood the flu, as a pathological entity, in the early twentieth century. For this, we examine some of the cognitive resources that doctors used to address and respond to the flu epidemic of 1918 in Bogota. In this approach, we can perceive how the flu epidemic became a social phenomenon of enormous complexity and how its medical definition was forged in a field of debate that pitted doctors who were impacted by the major paradigm shifts undergone by medicine during the late nineteenth century. The text explores some of the central concepts that served as a basis for the etiologic understanding of the flu, presents a clinical description of the clinical knowledge about the disease available at the time, and reviews in detail the debate that led to the identification of a specific microorganism as a causal agent of the flu.