Both these children (died during a Diphtheria Epidemic) Diphtheria is a contagious disease spread by direct physical contact or breathing the aerosolized secretions of infected individuals. An upper respiratory tract illness caused by Corynebacterium diphtheriae, a facultative anaerobic, Gram-positive bacterium. Historically quite common, diphtheria has largely been eradicated in industrialized nations through widespread vaccination.