Among well-educated, comfortably off parents, the ranks of vaccine-resistors are increasing. (Of course, plenty of parents fail to vaccinate their kids not by choice, but because they're poor and lack access to decent healthcare.) Some states with a large number of skeptical, alternative-minded people — Massachusetts, Oregon, Washington, New York — have seen, in the past six years, a declining percentage of children vaccinated against polio,Vaccine resistance bears the familiar markers of Generation X parenthood. diptheria, measles, mumps and rubella. Of the school requirements, my playgroup hostess may not be far off the mark: it is becoming easier in many states to opt out of vaccinations. I spoke with many parents who were planning to get religious exemptions — which in New York State are easy to come by, even if you're an atheist (all states except Mississippi and West Virginia permit parents to opt out on religious grounds, according to the National Vaccine Information Center, an anti-vaccination group). Eighteen states permit opting out for "philosophical, personal or conscientiously held" beliefs; most of these laws were passed in the last decade. Texas has made it even easier to avoid vaccines: in 2005 the state enacted a law permitting a parent to opt out for any reason.

Though parents who don't vaccinate have usually done mountains of research, and are fluent in the language of evidence and science, they'll often admit that their decision isn't ultimately about the facts. Says Lorena, "my gut tells me. I just know that I will not vaccinate my child." A pediatrician in the Northeast who blogs anonymously on these issues — and accepts non-vaccinators in his practice — tells me that worried inquiries from parents about the autism link have increased, probably because of media coverage. Some people do have bad reactions to vaccines, this doctor acknowledges, "but hysteria is never a good thing. And we're at near-hysteria right now."

Why now? Partly because, although there have been vaccine skeptics for as long as there have been vaccines, they tend to be most vocal when major childhood diseases are in retreat (not many Africans are worrying about autism right now). Side effects from vaccines are not unknown, of course, and since few of us know anyone who has had a vaccine-preventable disease, more and more parents question the risk. Diseases like polio, says Dr. Bernstein, are "out of sight, out of mind, so many parents may not feel t