Abstract Diphtheria is a potentially lethal infection caused by toxin-producing strains of Corynebacterium diphtheria, a Gram-positive bacillus. Humans are the only known reservoir, with spread via respiratory droplets or direct contact with skin lesions. Although now rare in developed countries, this vaccine-preventable disease remains an important problem in countries with poor or failing health systems, and is estimated to cause about 5,000 deaths per year worldwide, most in children under 5 years of age. Diphtheria develops when toxigenic bacteria lodge in the upper airway or on the skin of a susceptible individual. An intense inflammatory reaction develops, leading to a characteristic greyish-coloured pseudomembrane that is adherent to underlying tissues. Systemic effects are caused by release of diphtheria toxin, carried by a lysogenic corynebacteriophage, a single molecule of factor A of which can kill a eukaryotic cell.