Diphtheria is an acute infectious disease caused by the bacteria Corynebacterium diphtheriae.

Diphtheria spreads through respiratory droplets (such as those produced by a cough or sneeze) of an infected person or someone who carries the bacteria but has no symptoms. Diphtheria can also be spread by contaminated objects or foods (such as contaminated milk).

The bacteria most commonly infects the nose and throat. The throat infection causes a gray to black, tough, fiber-like covering, which can block the airways. In some cases, diphtheria may first infect the skin, producing skin lesions.

Once infected, dangerous substances called toxins, produced by the bacteria, can spread through your bloodstream to other organs, such as the heart, and cause significant damage.

Because of widespread and routine childhood DPT immunizations, diphtheria is now rare in many parts of the world. There are fewer than five cases of diphtheria a year in the United States.

Risk factors include crowded environments, poor hygiene, and lack of immunization.