This article examines the history of diphtheria in the Yukon and the Mackenzie district of the Northwest Territories in the first half of the 20th century. This analysis follows the traces of this now largely forgotten disease and its treatment to illuminate the constraints - intrinsic and constructed - on the provision of health care commensurate with the expectations and needs of northern Indigenous peoples. While diphtheria was never the most serious infectious disease, nor a major cause of death compared with tuberculosis or influenza at this time, examining its history offers significant insight into the creation of medical and public health infrastructures in Canada's northern territories, and the ways in which those infrastructures served, and failed to serve, different northern populations.