An acute infectious disease of humans caused by Corynebacterium diphtheriae. Classically, the disease is characterized by low-grade fever, sore throat, and a pseudomembrane covering the tonsils and pharynx. Complications such as inflammation of the heart, paralysis, and even death may occur due to exotoxins elaborated by toxigenic strains of the bacteria. The upper respiratory tract is the most common portal of entry for C. diphtheriae. It can also invade the skin and, more rarely, the genitalia, eye, or middle ear. The disease has an insidious onset after a usual incubation period of 2–5 days.

The only specific therapy is diphtheria antitoxin, administered in doses proportional to the severity of the disease. Antitoxin is produced by hyperimmunizing horses with diphtheria toxoid and toxin. It is effective only if administered prior to the binding of circulating toxin to target cells. Antibiotics do not alter the course, the incidence of complications, or the outcome of diphtheria, but are used to eliminate the organism from the patient.

Persons with protective antitoxin titers may become infected with diphtheria but do not develop severe disease. Since the 1920s, active immunization with diphtheria toxoid has proved safe and effective in preventing diphtheria in many countries. Diphtheria toxoid is produced by incubating the toxin with formalin. Active immunization requires a primary series of four doses, usually at 2, 4, 6, and 18 months of age, followed by a booster at school entry. Medical bacteriology Toxin