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. Diphtheria has been known since ancient times: Hippocrates first described the disease in the fifth century BC.
Diphtheria starts 1 to 10 days after a person is infected; most often within 2 to 5 days of infection. Usually the infection is in the throat and/or on the tonsils, but any mucous membrane can be infected. Less common sites of infection include the nose (an infection here can look almost exactly like the common cold, except that we can see a white membrane on the mumous membranes inside the nose, especially on the nasal septum (the cartilage wall between the two nostrils). The diphtheria toxins are usually not absorbed well from the nose, and so the disease ends up being less severe. The larynx (voice box) can also be infected; this is dangerous since the membrane can easily block the airway. It's also possible to be infected with C. diphtheriae on the conjunctivae (the clear membrane covering the surface of the eye and the inside of the eyelids, on the genital mucous membranes, and even on intact skin. Skin infection with C. diphtheriae is common in tropical regions, and often seen in homeless people in the United States; the infection may appear as a skin ulcer or as a scaly rash.
The most serious effects of diphtheria toxin are on the heart (muscle damage leading to loss of pumping ability), kidneys, and the nervous system. There are strains of C. diphtheriae that do not produce toxin; these can be distinguished in the laboratory. The toxin must be absorbed by the body to affect someone; this is why nasal and skin infections are often less severe than airway infections. 5-10% of patients with diphtheria die of the disease, but the rate is up to 20% for infected people less than 5 years old or more than 40 years old.
Diphtheria can be treated by giving penicillin or erythromycin to kill the bacteria, and antitoxin to clear free toxin in the body. 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 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.