Diphtheria is a human disease caused by Corynebacterium diphtheriae, so named for its clubbed shape (Greek: koryne, or “club”) and for the hidelike pseudomembrane (Greek: diphtheria, for “shield” or “membrane”) that forms on the tonsils, palate, or pharynx in severe cases of infection. Although this bacillus may cause no more than an innocent, subclinical infection and can be transmitted via well carriers, during diphtheria epidemics more virulent strains are responsible for case fatality rates ranging from 30 to 50 percent of affected young children. In such circumstances the bacterium itself is infected by a phage virus responsible for the elaboration of a potent exotoxin. Even though the exotoxin can cause rapid fatty degeneration of the heart muscle and peripheral nervous system damage resulting in paralysis, young children often die because the airway is occluded. Both the suddenness of suffocation in children and the capricious emergence of virulent epidemics of diphtheria are important features of historical interest in the disease.

Also called the Klebs-Löffler bacillus in early twentieth-century medical literature, this gram-positive organism is usually spread by respiratory secretions and droplet infection. After a brief incubation period of 2 to 4 days, the bacillus multiplies in the upper respiratory tract, creating a membranous exudate on pharyngeal tissues. The bacillus invades the local tissues and kills cells, causing necrosis and, often, discoloration of the membrane. The foul breath associated with necrosis and the greenish or blackened membrane are hallmarks of the disease to most clinical observers differentiating diphtheria from streptococcal sore throat and from croup.