Diphtheria, dihf THIHR ee uh or dihp THIHR ee uh, is a severe, contagious infection of the upper respiratory system or the skin. It can involve serious--or even fatal--complications. During the late 1800's, diphtheria epidemics swept the United States and Western Europe. At that time, most victims were under 10 years of age. Today, diphtheria affects children and adults about equally. Widespread immunization with diphtheria vaccines, which first came into use about 1920, has greatly reduced the number of cases of diphtheria.