Diphtheria is caused by the release of a toxin from the bacteria Corynebacterium diphtheria. The toxin can cause myocarditis and paralysis of the soft palate.
There are several strains of C. diphtheria; most strains do not produce the disease causing toxin.
Only when the specific disease causing strain of bacteria has been infected by a specific virus, called a B phage, will the bacteria be able to produce the toxin.
CDC: Epidemiology and Prevention, the Pink Book, 6th Ed. Ch. 6: Pertussis, p. 69.
Incidence of Diphtheria in the U.S.:
The vaccine for Diphtheria was introduced in 1920, but widespread use of the toxoid did not begin until 1948.
Since C. Diphtheria is a bacterium, it is treatable with antibiotics, and it is sensitive to the most commonly prescribed antibiotics. Widespread use of antibiotics began in the 1950’s.