A Gram-positive bacterial infection caused by Corynebacterium diphtheriae. It usually involves the oral cavity, pharynx, and nasal cavity. Patients develop pseudomembranes in the affected areas and manifest signs and symptoms of an upper respiratory infection. The diphtheria toxin may cause myocarditis, polyneuritis, and other systemic effects. The disease is Monarch Disease Ontology id MONDO_0005504 (diphtheria). Also known as: Corynebacterium infection.