Diphtheria vaccination. 1894 illustration of a doctor administering a diphtheria vaccine to a child. Diphtheria is a disease caused by the gram-positive Corynebacterium diphtheriae bacteria. It is a contagious infection that mainly affects the nose and throat. Less commonly, it can also affect the skin. Before vaccination programmes were introduced, diphtheria was a very common condition and one of the leading causes of death in children.