Systemic intoxication is caused by diphtheria exotoxin, an extracellular protein metabolite of toxigenic strains of C. diphtheriae . Protection against disease is due to the development of neutralizing antibodies to diphtheria toxin. At one time, diphtheria was common in the United States. More than 200,000 cases, primarily among young children, were reported in 1921. After 1979, cutaneous diphtheria was no longer a notifiable disease.
Most cases occur among unvaccinated or inadequately immunized persons.
Both toxigenic and nontoxigenic strains of C. diphtheriaecan cause disease, but only strains that produce toxin cause myocarditis and neuritis. A complete vaccination series substantially reduces the risk of developing diphtheria, and vaccinated persons who develop disease have milder illness. Protection lasts at least 10 years. Vaccination does not, however, eliminate carriage of C. diphtheriae in the pharynx or nose or on the skin. The occurrence of tetanus in the United States has decreased dramatically from 560 reported cases in 1947 to a record low of 48 reported cases in 1987.