Diphtheria (dif-THEER-e-uh) is a serious bacterial infection that usually affects the mucous membranes of the nose, eyes, skin, and throat. The online search query, "What is diphtheria?", or "What is croup?", can be answered as "a disease caused by the microorganism *Corynebacterium diphtheriae* colonizing these areas." Thanks to widespread vaccination, diphtheria is extremely rare in Turkey and other developed countries. However, many countries with limited healthcare access or vaccination options still experience high rates of diphtheria. Diphtheria is treatable with medication. However, in advanced stages, it can damage the heart, kidneys, and nervous system. Even with treatment, diphtheria can be fatal, especially in children. Diphtheria was discovered in 1826 by the scientist Pierre Bretonneau. The diphtheria vaccine began to be used in 1923. It is a disease that affects children and is more common during colder months. Before the age of 2, it only appears as nasal and wound diphtheria. In adults, it is milder than in children. In the United States, it peaked in 1921 with 206,000 cases. Currently, it is most prevalent in Asia, the Dominican Republic, Haiti, the South Pacific, and the Middle East.