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On Feb. 23, 1954, Dr. Jonas Salk began administering his experimental vaccine against polio to a selected sample of children in Pittsburgh, Pa. A "planned miracle" was about to take place, and a terrible curse was about to be lifted from the face of the Earth.

Children today have no idea of what it was like to live in fear of the crippler known as poliomyelitis, or "infantile paralysis." Only a few decades ago, however, summertime was thought of as "polio season," and parents tried to keep their children away from public swimming pools and other public places, hoping that medical science and the March of Dimes would somehow find a cure for this nightmare called polio. Everybody knew somebody - a relative, a classmate or a kid down the block - who had contracted the awful disease and would never walk again.Polio had actually been around for thousands of years, but it was the epidemics - the tens of thousands of cases in a single year - that were new to the 20th century. Before the introduction of modern plumbing and sterilized baby bottles, children were exposed to a wide range of germs at an early age, including the polio virus, and they developed a lifelong immunity to it. But modern sanitation prevented children from developing this immunity, and so severe polio became an unintended consequence of modern public health.

What was needed, then, was some way to expose children to polio, so that their blood could produce antibodies that would attack the disease, without risking the possibility that the polio would ever defeat the body's defenses and make its way to the nerves in the spinal cord.

This process of stimulating the body's immune system by introducing harmful but weakened germs actually predates our knowledge that germs were the cause of infectious diseases. It was not that long ago that smallpox terrified our ancestors by killing perhaps one in 10 and leaving countless others lame or blind or disfigured with pock-marked skin (as it did George Washington). All that was known about this disease was that it almost never struck the same person twice, and so if you could be lucky enough to catch it in a mild form, you would never have to fear it again.

In the late 18th century, an English country doctor named Edward Jenner noticed that the milkmaids on the surrounding farms who had contracted a disease called cowpox (from the udders of infected cows) never came down with smallpox. Cowpox was a much milder disease than smallpox, and so Jenner decided to try injecting some fluid from a milkmaid's cowpox sore into an 8-year-old boy, and then seeing if the boy became immune to smallpox. Twenty times Jenner tried exposing him to smallpox, but the boy showed no symptoms whatsoever of the disease. Jenner proudly announced his discovery, and he named the process after the Latin word for cow, which was "vacca," calling it "vaccination" and the infecting agent itself a "vaccine."

In these columns, I have repeatedly stressed the need for children to learn about the wonders of the human body, especially the way the body struggles to keep us in good health. The 612 section of the children's collection in your local library has a number of books designed to explain the workings of the human body and to answer common questions that children have about how the body operates. Among my personal favorites are "How & Why," by Catherine O'Neill, and "Junior Body Machine," by Dr. Christiaan Barnard.