Brad Levine, age 11, of Las Vegas, Nev., for his question:

HOW DOES DIPHTHERIA SPREAD?

Diphtheria is a severe contagious disease in which a false membrane grows over a mucous membrane, usually in the throat or nose. Diphtheria is caused by a bacillus and it is spread from one person to another.

Because the bacteria usually live in the human nose and throat, doctors believe that the most common method for spreading the disease is by coughing and sneezing. Persons called carriers may harbor the bacteria without showing any signs of the disease. Such persons may spread the disease to others.

Most people can get diphtheria, but it is usually not found in infants. The disease usually strikes during either the autumn or winter. Fortunately, the number of diphtheria cases has sharply declined in recent years.

The strong false membrane of diphtheria appears as a yellowish gray patch on the mucous membrane of the victim's tonsils, pharynx, nose or throat. In rare cases, the membrane may form over an open skin wound.

When it forms in the throat, the membrane may obstruct the throat so that the victim cannot breathe properly. Sometimes a doctor will have to make a temporary opening through the neck, directly into the windpipe, to save the patient's life.

In most cases of diphtheria, the bacteria remains in the victim's throat and never enters the bloodstream. But the bacteria can produce a poison which the blood can carry to all parts of the body.

Doctors treat diphtheria by injecting an antitoxin into the patient's muscles. The antitoxin is a substance that neutralizes the toxin produced by the germs.

Diphtheria antitoxin is made by concentrating the blood serum of horses or sheep that have been inoculated repeatedly with diphtheria toxin.

The first effective diphtheria antitoxin was developed in 1890 by Emil von Behring, a German bacteriologist.

Diphtheria can be prevented. Doctors now give injections of a material that will help to stimulate a person's body to produce immune antibodies that help to prevent the bacteria's growth.

A test to determine a person's immunity to diphtheria was introduced by Dr. Bela Schick in 1914. Doctors used this test during outbreaks of diphtheria to determine which persons need inoculations.

Doctors also used Schick's test after an inoculation to make sure that the toxoid had made the person immune.

During the 1800s, diphtheria epidemics swept the United States and Western Europe. Since 1890, there haven't been too many epidemics.

Diphtheria is actually caused by a specific bacillus with this complicated name: Corynebacterium diphtheriae.