Diphtheria is caused by a bacterium, Corynebacterium diphtheriae, which typically infects mucous membranes: the nose and throat are favourite places for the infection to take hold, but mucous membranes of the eyes or genitalia can also be infected. The bacteria produce a toxin which causes damage to tissue both at the site of the original infection (the typical sign of diphtheria is a shaggy gray "membrane" on the back of the throat) and in other parts of the body once the toxin is spread via the bloodstream. The most serious effects of diphtheria toxin are on the heart (muscle damage leading to loss of pumping ability), kidneys, and the nervous system.
Diphtheria can be treated by giving penicillin or other antibiotics to kill the bacteria, and antitoxin to clear free toxin in the body. However the antitoxin will not clear toxin that has already bound to cells and started to damage them. It is nuch better to give toxoid to stimulate immunity to the toxin, thus enabling the body to clear toxin as soon as it appears. (Vaccinating against the bacteria itself is not possible as yet.) The toxoid is given initially at ages 2, 4, and 6 months, again at ages 18 months and 5 years, and regularly every 10 years after that. If this schedule seems familiar, it should: it's the same schedule as that for tetanus immunization, which is also done with a toxoid, and in fact the tetanus and diphtheria toxoids are routinely combined and given to babies together along with the vaccine against whooping cough (which is also given to babies on the same schedule). Adults usually have been given Td, which contains slightly less diphtheria toxoid than small children need and receive. Tdap, the new vaccine against whooping cough, also contains diphtheria toxoid in the (smaller) adult dose.