Diphtheria is an infectious disease caused by the gram-positive bacillus Corynebacterium diphtheriae. Infection may lead to respiratory disease, cutaneous disease, or an asymptomatic carrier state. The word diphtheria comes from the Greek word for leather, which refers to the tough pharyngeal membrane that is the clinical hallmark of infection. Rarely, a similar disease can be caused by other Corynebacterium species: C. ulcerans, C. hemolyticum, and C. pseudotuberculosis.
The epidemiology, pathophysiology, and clinical manifestations of diphtheria will be reviewed here. The diagnosis, treatment, and prevention are discussed separately. (See "Diagnosis and treatment of diphtheria" and "Standard immunizations for children and adolescents", section on 'Diphtheria, tetanus, and/or pertussis vaccines' and "Tetanus-diphtheria toxoid vaccination in adults".)
There are at least four biotypes of Corynebacterium diphtheriae: gravis, intermedius, mitis, and belfanti. All biotypes have been associated with both endemic and epidemic diphtheria, although, in general, mitis strains are less toxigenic and cause less severe disease .
The primary modes of spread consist of close contact with infectious material from respiratory secretions (direct or via airborne droplet) or from skin lesions. Humans are believed to be the only known reservoir for C. diphtheriae, although transmission of C. ulcerans via cow’s milk has been observed [2,3]. Infection may occur throughout the year with a peak incidence in the colder months.
Asymptomatic carriers are important for transmission of diphtheria. Immunity (either via natural infection or vaccine-induced) does not prevent carriage [4,5]. In areas of endemicity, up to 5 percent of healthy individuals may have positive pharyngeal cultures .