Meredith Schweikhart, age 14, of Missoula, Mont., for her question:

WHAT DOES IMMUNIZATION DO?

In preventive medicine, immunization is the process of rendering people immune to an infectious organism by inoculating them with a form of the organism that does not cause severe disease but does provoke formation of protective antibodies.

The process is also called vaccination, because the first instance of immunization was the use of vaccinia, or cowpox, virus to produce immunity to variola, or smallpox. This was first done by a British physician named Edward Jenner in 1796.

Vaccinations are the most effective protection against most diseases caused by viruses and related organisms, because few antibiotics work against them. In Western countries vaccines are routinely used in the first years of life to produce immunity to diphtheria, tetanus, poliomyelitis and whooping cough.

A vaccine may contain organisms killed by exposure to heat or chemicals, such as those for poliomyelitis and typhoid. It may contain an inactive form of a toxin, produced by the organism and called a toxoid, as in tetanus and diphtheria vaccines, or it may contain a live "attenuated" virus, which has been grown in such a way that it can no longer cause serious disease, as in the vaccine against polio developed by Albert Sabin, and in vaccines against measles and yellow fever.

In 1885 the great French scientist Louis Pasteur first used an attenuated rabies virus to protect against the natural infection, and in 1897 a vaccine against typhoid fever was developed in England.

The immunizing substance is usually introduced through a scrape in the skin, called inoculation, although the Sabin polio vaccine is taken orally on a sugar cube. Protection provided by vaccines lasts for varying periods.                                         

A population can be immunized in two ways. In one method, the vaccine is targeted to those who are most likely to get the disease. In the recent successful campaign to eradicate smallpox worldwide, a form of this strategy was used.

Most diseases in Western countries, however, are controlled through the principle of herd immunity, in which it is held that the transmission of disease will be stopped when an extremely low probability exists that an infected person will come into contact with an unprotected individual. Not every person need be immunized, but protection levels of 90 percent must be reached for some diseases.

In some instances a combined strategy is used. For rubella, or German measles, for example, public health workers aim at mass immunization of school age children as well as immunization of women of childbearing age.

Immunization remains one of the primary weapons in the fight against infectious disease.