American badgers (Taxidea taxus jacksoni) at the periphery of the species' range in Ontario, Canada, are listed as endangered because of an estimated population size of <200 mature individuals. The main threats faced by this population include habitat loss and road mortality. However, on 18 November 2013, a radio-implanted badger was found nonresponsive in an agricultural field with signs consistent with canine distemper virus infection, which was subsequently confirmed. This prompted our investigation into the occurrence of pathogens in this endangered carnivore to better quantify the level of risk infectious disease poses to population persistence. We examined serum samples from nine live-trapped individuals and 27 whole badger specimens submitted for postmortem examination. We found evidence of exposure to canine distemper virus, canine parvovirus, and leptospires. However, infection associated with disease was not the leading cause of mortality. Future research into the effects of disease on kit survival and a comprehensive understanding of disease severity and spread from reservoir populations (e.g., raccoons [Procyon lotor] and striped skunks [Mephitis mephitis]) to badgers will be of particular importance to the conservation of this endangered population.