At the end of the 19th century, diphtheria was one of the infectious diseases striking children the most. With the development of laboratory medicine, appearing at this time throughout Europe, came the identification of the diphtherial bacillus, which not only causes a local infection, but also produces a poison attacking the whole organism. The discovery of this microbe will be an important vector for bacteriological research, leading to the elaboration of the antidiphtherial serum, considered the first specific and effective therapeutic product in the new experimental medicine. Through the study of diphtheria in the canton of Geneva, we examine the causes and consequences of the application of laboratory techniques and concepts in the sphere of clinical medicine.