The bacteria that cause diphtheria spread through respiratory droplets (such as from a cough or sneeze) of an infected person or someone who carries the bacteria but has no symptoms.
The bacteria most commonly infects your nose and throat. The throat infection causes a gray to black, tough, fiber-like covering, which can block your airways. In some cases, diphtheria infects your skin first and causes skin lesions.
Once you are infected, the bacteria make dangerous substances called toxins. The toxins spread through your bloodstream to other organs, such as the heart and brain, and cause damage.
Because of widespread vaccination (immunization) of children, diphtheria is now rare in many parts of the world.
Risk factors for diphtheria include crowded environments, poor hygiene, and lack of immunization.