This thesis examines the history of diphtheria in Ontario between 1880 and 1940. The purpose of this thesis is to look past the bacteriological excitement of the nineteenth century, and the discoveries that have often been reported by historians and popular media and explore why diphtheria remained an enigmatic disease despite the discovery of a single bacterial cause. Drawing on a variety of primary and secondary sources, this thesis also uncovers the social, personal and often fatal consequences that arose following the appearance of diphtheria within communities. The unresolved enigmatic nature of diphtheria allowed for the creation of a conceptual space in which both medical and non-medical members of Ontario’s society often found themselves competing to promote their own conceptualizations of diphtheria. These conceptualizations, combined with the threat diphtheria posed to the health of a community, resulted in further confusion regarding the nature of the disease. Many historical concerns regarding diphtheria and its enigmatic nature have never been resolved.