Diphtheria is caused by the bacterium *Corynebacterium diphtheriae*. The bacteria usually multiply on or near the surface of the throat or skin. Diphtheria spreads through airborne droplets. People nearby can inhale *C. diphtheriae* when an infected person sneezes or coughs. Diphtheria spreads easily this way, especially in crowded environments. Additionally, diphtheria can be transmitted through contaminated personal or household items lacking hygiene. People can sometimes contract diphtheria when handling items that may be contaminated with the bacteria, such as used utensils or towels from an infected person. Touching an infected wound can also transfer the diphtheria-causing bacteria. Untreated individuals infected with *Corynebacterium diphtheriae*, even if asymptomatic, can spread diphtheria to unvaccinated individuals.


Individuals at higher risk of contracting diphtheria include unvaccinated children and adults, those living in crowded and unsanitary conditions, and those traveling to areas with prevalent whooping cough infections. Diphtheria is rarely seen in areas with widespread vaccination. However, it remains a prevalent disease in countries without widespread vaccination. In regions with standard diphtheria vaccination, the disease primarily poses a threat to unvaccinated or under-vaccinated individuals who are internationally traveling or in contact with people from less-developed countries.