From the Guidelines

Diphtheria is a rare, acute, and communicable infectious illness caused by toxigenic strains of Corynebacterium diphtheriae.

Definition and Cause

Diphtheria is caused by toxigenic strains of Corynebacterium diphtheriae, which are nonmotile, nonencapsulated, club-shaped, Gram-positive bacilli 1.
The classic feature of respiratory diphtheria is a gray-colored pseudomembrane that is firmly adherent to the mucosa lining the nasopharynx, tonsils, or larynx 1.

Vaccination with diphtheria toxoid-containing vaccines prevents diphtheria 1.
A complete vaccination series substantially reduces the risk of developing diphtheria, and vaccinated persons who develop disease have milder illnesses 1.
The effectiveness of diphtheria toxoid is high, although not 100%, with virtually all infants developing diphtheria antitoxin titers >0.01 IU/mL after receiving 3 doses of diphtheria toxoid–containing vaccines 1.

Diphtheria is a nationally notifiable disease in the United States, with reported cases declining from approximately 200,000 in 1921 to 15,536 in 1940, and only 13 cases reported between 1996-2016 1.
Although childhood DTaP vaccination coverage is >80% in the United States, immunity acquired from childhood vaccination wanes in the absence of decennial boosters, and older adults might not be adequately protected 1.
Exposure to diphtheria remains possible during travel to countries with endemic disease or from imported cases 1.

Diphtheria is a potentially fatal infection mostly caused by toxigenic Corynebacterium diphtheriae strains and occasionally by toxigenic C. ulcerans and C. pseudotuberculosis strains 2.
It is an acute respiratory infection, characterized by the formation of a pseudomembrane in the throat, but cutaneous infections are possible 2, 3.
Systemic effects, such as myocarditis and neuropathy, which are associated with increased fatality risk, are due to diphtheria toxin, an exotoxin produced by the pathogen that inhibits protein synthesis and causes cell death 2.

The most common sites of infection are the pharynx and the tonsils, with common clinical manifestations that include sore throat, malaise, cervical lymphadenopathy, and low-grade fever 3.
Clinical diagnosis is confirmed by the isolation and identification of the causative Corynebacterium spp., usually by bacterial culture followed by enzymatic and toxin detection tests 2.
Diagnosis of diphtheria is primarily clinical, supported by demonstration of toxin-producing bacteria by culture 4.

The cornerstone of treatment for diphtheria is diphtheria antitoxin, and early treatment is critical as the degree of protection is inversely proportional to the duration of the illness before its administration 3.
Diphtheria can be treated with the timely administration of diphtheria antitoxin and antimicrobial therapy 2.
Routine childhood vaccination virtually eliminated diphtheria in most industrialized countries, but universal immunization, early diagnosis, prophylaxis, and adequate supportive care are measures to prevent it 3, 4.
Improved vaccination coverage, including booster dosage, coupled with early detection and effective treatment, may all reduce incidence and mortality 5.