Millennial Americans have to be the world's most aggressively adventurous human beings. We eagerly rappel down rockfaces, canyoneer across rivers, and paraglide over mountaintops. But those are risks we choose to take. We adopt a very different attitude toward the risks of daily life, no matter how small, which we can't avoid. Although most tap water is safe to drink, we spend millions on bottled brands. Despite declining rates of many major cancers, we are obsessed by our fear of the disease, and of the pesticides, sun, and any food, chemical, or place that somebody suspects might cause it.

So it is not surprising that a growing movement of worried parents, believers in alternative medicine, and political libertarians has emerged to question the safety of the vaccinations that all children are required to have before entering school. Fuelled by warnings on the Internet, their fears are based on the rare adverse reaction that vaccines can cause. And adverse reactions do occur. For example, the Centers for Disease Control has called for an end to the use of the oral polio vaccine, which, because it contains weakened but live virus, triggered the disease in about ten children out of the millions who took it each year. (A newer version eliminates this risk.) And use of the rotavirus vaccine was recently suspended after reports that it caused bowel obstruction in infants.

This summer, the protests persuaded Dan Burton, the melon-shooting Indiana congressman, to convene hearings on vaccine safety, and more are planned for next year. The testimony from parents whose children had suffered violent reactions was passionate and moving. But, for anyone who followed these sessions, one fact always managed to bob above the waves of pseudoscientific argument: childhood vaccinations are the most effective public-health measure in American history.

Most people can't remember a time when polio, measles, diphtheria, and smallpox killed tens of thousands of children each year. But it wasn't long ago. From 1951 to 1954, an average of sixteen thousand children became ill with paralytic polio each year. When the Salk vaccine was introduced, in 1955, the disease began to recede, and it has been twenty years since a live polio transmission was documented in the United States. Half a million cases of measles were reported each year in the early sixties just before a vaccine became available and thousands of children died. Last year, there were a hundred cases of measles; none were fatal.

So why has doubt arisen about such a fundamental good? A lack of confidence in public-health policy is certainly part of the reason. But so, ironically, is the remarkable success of vaccines, which has left parents who have never seen a case of polio or measles to focus their attention solely on the failures. Bruce Gellin, a specialist in infectious diseases at Vanderbilt University medical school, was so alarmed by their disaffection that last year he created the National Immunization Information Network. With the help of such medical associations as the American Academy of Pediatrics and the Infectious Diseases Society of America, the network attempts to provide scientifically sound information about vaccines. "When a child is hurt by something that is supposed to protect him, the impact is devastating," Gellin says. "But we seem to live in a society where no risk is the only risk that is acceptable. Our perceptions have all been skewed."

That's putting it mildly. About three hundred people will drown in their bathtubs this year in the United States; the rest will probably keep bathing. A thousand Americans will choke to death on food, but it won't alter the nation's eating patterns. Yet vaccine opponents apparently would rather imperil the population than accede to the government mandate.

The most influential advocacy group in the campaign against universal vaccination, the National Vaccine Information Center, based in suburban Virginia, is trying to take the debate farther. The organization's president, Barbara Loe Fisher, believes that her son suffered permanent damage from an old version of the diphtheria-pertussis-tetanus vaccine which was associated in rare cases with high fevers and convulsions. She maintains that the rise in rates of diabetes, asthma, and even autism over the past three decades may be associated with the growth in the number of vaccines, which, she argues, may weaken a child's immune system. There is no scientific evidence for any such notion. One might as well say that the rise in vaccines since 1960 has caused the increase in the rate of divorce.

Fisher also suggests that since measles has nearly been eradicated in America we no longer need to vaccinate every child. That would be a dangerous mistake: when the measles-vaccination rate dropped in 1989, the result was a two-year epidemic. (A second shot is now required.) "The one-size- fits-all approach to mass vaccination has not been intelligent, rational, or scientifically sound,'' Fisher said recently. "Our grandparents got one vaccination–the smallpox. Now our kids get thirty-three doses of ten vaccines by the age of five."

Has there ever been a clearer definition of scientific progress?