Diphtheria is an acute, respiratory infection caused by Corynebacterium diphtheriae, a gram-positive bacterium. The causative organism produces a potent exotoxin responsible for localised tissue destruction and formation of pseudomembrane in the upper respiratory tract. While the organism itself is rarely invasive, the exotoxin produced by toxigenic strains can exert both local and systemic effects. Haematogenous dissemination of the toxin may lead to serious complications, such as myocarditis and neuritis. Pharyngeal diphtheria, the most common manifestation, can result in acute and potentially life-threatening airway obstruction.

The disease is transmitted primarily through respiratory droplets and by direct contact with skin lesions or contaminated objects. Infection most often occurs following close contact with symptomatic patients or asymptomatic carriers. Notably, individuals with natural or vaccine-induced immunity may still harbour the organism and contribute to its spread.

In Malaysia, diphtheria incidence declined markedly after the introduction of routine childhood immunisation. Since 2016, diphtheria cases in Malaysia peaked and later declined, reaching their lowest during the COVID-19 pandemic. However, numbers began to rise again after 2022, showing a continuing upward trend.