Diphtheria is a contagious infectious disease that primarily affects the upper respiratory tract (respiratory diphtheria), and it is characterized by sore throat, fever, and an adherent membrane (pseudomembrane) on the tonsils and nasopharynx. Diphtheria can also affect the skin and cause localized skin infections (cutaneous diphtheria). Severe infection with diphtheria can lead to systemic involvement and can affect other organ systems as well, such as the heart and nervous system, sometimes leading to death. Diphtheria is caused by the bacterium Corynebacterium diphtheriae.

Diphtheria was first described by Hippocrates in the fifth century BC, and throughout history diphtheria has been a leading cause of death, primarily among children. The diphtheria bacterium was first identified in the 1880s by F. Loeffler, and the antitoxin against diphtheria was later developed in the 1890s. The development of the first diphtheria toxoid vaccine occurred in the 1920s, and its subsequent widespread use led to a dramatic decrease of diphtheria worldwide.

Though the implementation of vaccination programs has significantly decreased the incidence of diphtheria, serious outbreaks may still occur when vaccination rates wane. One such outbreak occurred in the 1990s in the Russian Federation and the Newly Independent States of the former Soviet Union, in which the World Health Organization (WHO) reported more than 157,000 cases and 5,000 deaths. Though still endemic in many parts of the world, respiratory diphtheria in the United States is currently a rare disease that has largely been eliminated through effective vaccination programs.