Diphtheria is a disease caused by Corynebacterium diphtheriae, a pleomorphic gram-positive bacillus the virulence of which is largely attributed to its potent exotoxin.

The clinical manifestation of diphtheria typically involves the respiratory tract. Following an incubation period of 2-4 days, patients typically experience abrupt onset of fever, malaise, sore throat, and the development of an adherent, gray-brown, pseudomembranous exudate around the tonsils, nares, pharynx, larynx, and/or tracheobronchial tree. The presence of the exudate, coupled with soft tissue edema around the respiratory tract, can cause hoarseness, dyspnea, stridor, and cough; these symptoms can progress to respiratory failure, and death may occur due to suffocation after aspiration of the membrane. Because of the activity of the diphtheria toxin, localized infection may lead to delayed systemic complications, including myocarditis, conduction defects (first-degree heart block progressing to atrioventricular dissociation and other arrhythmias), and cranial and sensorimotor peripheral neuropathy. The case fatality rate for respiratory diphtheria is approximately 10%. On rare occasions, an illness similar to respiratory diphtheria is caused by other Corynebacterium species (Corynebacterium ulcerans or Corynebacterium pseudotuberculosis) that produces the diphtheria toxin.

Other manifestations of diphtheria include chronic indolent skin ulcers bearing gray membranes and rare invasive infections such as septic arthritis, bacteremia, and endocarditis that are caused by non-toxigenic C diphtheriae.

The only known reservoirs of C diphtheriae are humans, and the primary modes of transmission are via respiratory droplets or direct contact with respiratory secretions or exudates from skin lesions. Asymptomatic respiratory carriage of C diphtheriae has been significantly reduced by the advent of widespread vaccination practices. However, lapses in vaccination and socioeconomic changes in some parts of the world have led to the re-emergence of diphtheria outbreaks and epidemics.

Pediatric patient considerations:
Compared with the adult respiratory tract, the proportionally smaller pediatric airway may be more rapidly compromised with pseudomembrane formation and surrounding cervical adenitis and soft tissue edema.