What is the definition or description of: diphtheria?

A bacteria. Diphtheria is a potentially fatal, contagious disease that usually involves the nose, throat, and air passages, but may also infect the skin. Its most striking feature is the formation of a grayish membrane covering the tonsils and upper part of the throat. This is a preventable disease by vaccination. It is rare in United States.
DIPHTHERIA. Infection caused by the bacterium c. Diphtheriae and its highly potent toxin; marked by severe inflammation (redness) thick exudate (pus) of the mucous membrane of the pharynx, nose, and sometimes the tracheobronchial tree. The toxin produces degeneration in peripheral nerves, heart muscle, and other tissues. Diphtheria is fatal, especially in children, but now rare due to an effective vaccine.