Diphtheria is a serious upper respiratory tract bacterial infection, usually affecting the mucous membranes of your nose and throat. Diphtheria is caused by Corynebacterium diphtheriae. Diphtheria is a bacterial infection that spreads easily and occurs quickly. Diphtheria is highly contagious. It’s easily passed from the infected person to others through sneezing, coughing, or even laughing. It can also be spread to others who pick up tissues or drinking glasses that have been used by the infected person.