Diphtheria is caused by toxigenic strains of the gram-positive bacteria Corynebacterium diphtheriae and Corynebacterium ulcerans, and affects the upper respiratory tract and occasionally the skin. The infection is transmitted via airborne droplets, generally requiring close contact with symptomatic patients or asymptomatic carriers. Alternative modes of transmission are direct contact with skin lesions or infected cattle and other farm animals (C. ulcerans). Rare human cases have been associated with the consumption of raw unpasteurized dairy products. Life-threatening complications include cardiac failure and paralysis.