Diphtheria is caused by Corynebacterium species, mostly by toxin-producing Corynebacterium diphtheriae and rarely by toxin-producing strains of C. ulcerans and C. pseudotuberculosis. The most common type of diphtheria is classic respiratory diphtheria, whereby the exotoxin produced characteristically causes the formation of a pseudomembrane in the upper respiratory tract and damages other organs, usually the myocardium and peripheral nerves. Acute respiratory obstruction, acute systemic toxicity, myocarditis and neurologic complications are the usual causes of death. The infection can also affect the skin (cutaneous diphtheria). More rarely, it can affect mucous membranes at other non-respiratory sites, such as genitalia and conjunctiva. C. diphtheriae is transmitted from person to person by intimate respiratory and direct contact; in contrast, C. ulcerans and C. pseudotuberculosis are zoonotic infections, not transmitted person-to-person. The incubation period of C. diphtheriae is two to five days (range 1– 10 days). A person is infectious as long as virulent bacteria are present in respiratory secretions, usually two weeks without antibiotics, and seldom more than six weeks. In rare cases, chronic carriers may shed organisms for six months or more. Skin lesions are often chronic and infectious for longer periods. Effective antibiotic therapy (penicillin or erythromycin) promptly terminates shedding in about one or two days.