Diphtheria: An acute infectious upper respiratory tract disease that affects the throat. It is caused by the bacteria Corynebacterium diphtheriae. Symptoms include sore throat and mild fever at first. As the disease progresses, a membranous substance forms in the throat that makes it difficult to breathe and swallow. Diphtheria can be deadly. It is one of the diseases that the DT (diphtheria-tetanus), DTP (diphtheria-tetanus-pertussis), and DTaP (diphtheria-tetanus-acellular-pertussis) vaccines are designed to prevent.