Diphtheria bacillus. False-colour transmission electron micrograph of Corynebacterium diphtheriae the causative agent of diphtheria. Diphtheria is an acute infection, normally of the throat, contracted from contact with a carrier by air- bourne infection. The bacillus affects an area of mucous membrane, causing the tissue to swell. The danger to the patient is two-fold: the swelling may form an obstruction to breathing, and the bacillus secretes a toxin which passes into the blood. The toxin may cause failure of the kidneys, heart muscles or local paralysis. Treatment is with an antitoxin; prevention is by immunization. Magnification x20, 000 at 6x4. 5cm size.