Corynebacterium diphtheriae is a Gram-positive pathogenic bacterium that causes diphtheria. It is also known as the Klebs–Löffler bacillus because it was discovered in 1884 by German bacteriologists Edwin Klebs (1834–1913) and Friedrich Löffler (1852–1915). These bacteria are usually harmless, unless they are infected by a bacteriophage carrying a gene which gives rise to a toxin. This toxin causes the disease. Diphtheria is caused by the adhesion and infiltration of the bacteria into the mucosal layers of the body, primarily affecting the respiratory tract and causing the subsequent release of an exotoxin. The toxin has a localized effect on skin lesions, as well as a metastatic, proteolytic effects on other organ systems in severe infections. Originally a major cause of childhood mortality, diphtheria has been almost entirely eradicated due to the vigorous administration of the diphtheria vaccination in the 1910s.
Diphtheria is no longer transmitted as frequently due to the development of the vaccine, DTaP. Although diphtheria outbreaks continue to occur, this is often in developing countries where the majority of the population is not vaccinated.