Diphtheria is an acute, highly contagious bacterial disease causing inflammation of the mucous membranes, formation of a false membrane in the throat that hinders breathing and swallowing, and potentially fatal heart and nerve damage by a bacterial toxin in the blood. It is now rare in developed countries because of immunization. This data includes confirmed diphtheria cases, including those confirmed clinically, epidemiologically, or by laboratory investigation. Cases that have been discarded following laboratory investigation should not be included.
Source: World Health Organization