One of Ludwik Fleck’s ideas about the development of scientific knowledge is that—once a system of interpretation is in place—the process that follows can be characterised as one of inertia: any new evidence comes under a strong pressure to be incorporated into the established frame. This can result in what Fleck called a harmony of illusions when contradictory evidence becomes almost invisible or is incorporated into the established frame only by huge efforts.The paper analyses early explanations of the tuberculin reaction (...) as a case study of Fleck’s argument. For Robert Koch, who had presented tuberculin in 1890, the compound was supposed to be a diagnostic tool and a cure for tuberculosis. His conception of its effect was rather peculiar, but strictly in line with ideas on the pathogenesis of infectious diseases he had developed much earlier. After tuberculin was released in late 1890, whether Koch’s conception was convincing depended on the place that a given observer had in the medical world in late-nineteenth-century Germany. Inside Koch’s group, the status of the tuberculin reaction remained stable and tuberculin retained its value as a diagnostic and curative tool. On the other hand, observers from outside that thought collective, and in particular from clinical medicine, soon pointed to flaws in its conception. These critics developed a rather different picture of tuberculin as a mysterious and dangerous drug. No reconciliation followed and what we find instead in German medicine around the year 1900 is the presence of rather contradictory concepts and practices surrounding Koch’s wonder cure. (shrink)