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But back on the Oprah show, McCarthy's charges went virtually unchallenged. Oprah praised McCarthy's bravery and plugged her book, but did not invite a physician or scientist to explain to her audience the many studies that contradict the vaccines-autism link. Instead, Oprah read a brief statement from the Centers for Disease Control saying there was no science to prove a connection and that the government was continuing to study the problem. But McCarthy got the last word. "My science is named Evan, and he's at home. That's my science." Oprah might say that McCarthy was just sharing her first-person story and that Oprah wasn't endorsing her point of view. But by the end of the show, the take-away message for any mother with young kids was pretty clear: be afraid.

Oprah told viewers that McCarthy would be available to answer questions and give guidance later that day on Oprah.com. One viewer went online to ask McCarthy what she would do if she could do it all over again. "If I had another child," McCarthy answered, "I would not vaccinate." A mother wrote in to say that she had decided not to give her child the MMR vaccine because of fears of autism. McCarthy was delighted. "I'm so proud you followed your mommy instinct," she wrote. A year later, McCarthy was back on the show for an episode about "Warrior Moms," which gave her another opportunity to expand on her claims about vaccines and autism. Oprah must have liked what she heard. McCarthy became a semiregular guest on the show, and in May, Oprah announced that her production company had signed McCarthy for a talk show of her own.

McCarthy is not the only guest who has warned Oprah's viewers off vaccines. Last summer Dr. Christiane Northrup, a physician and one of Oprah's regular experts, took questions from the audience. One woman asked about the HPV vaccine, which protects women against a sexually transmitted virus that can cause cervical cancer. Northrup advised against getting the shot. "I'm a little against my own profession," she said. "My own profession feels that everyone should be vaccinated." But Northrup cautioned, "There have been some deaths from the vaccine." She suggested a different approach. "Where I'd put my money is getting everybody on a dietary program that would enhance their immunity, and then they would be able to resist that sort of thing. All right?"

It is true that of the millions of women who have received the vaccine, 32 have died in the days or weeks afterward. But in each case, the Centers for Disease Control and the Food and Drug Administration investigated the deaths and found that they were coincidental and were not related to the shot. "This is a very safe vaccine," says Susan Wood, a research professor in the School of Public Health at George Washington University and the former head of the FDA's Office of Women's Health. "Because of the power and influence that Oprah's show has, she should make an extra effort to be clear." Neither did Oprah question Northrup's assertion that women can stop the spread of a cancer-causing sexually transmitted disease by eating healthy foods. There is, Wood says dryly, "no evidence that money spent on general health promotion" will do that. Reached by phone, Northrup herself now concedes she isn't certain that anyone has died from the vaccine. And she didn't mean to leave the impression that women should avoid it. "I would say that there is a chance that they could be injured from it, but I wouldn't say not to take it."

Northrup holds a special place in Oprah's constellation of regular guests. A Dartmouth-educated ob-gyn, she stresses alternative therapies and unseen connections between the soul and the body that she believes conventional doctors overlook, but that she can see. She has written about how she has used Tarot cards to help diagnose her own illnesses. (On her Web site, she sells her own "Women's Wisdom Healing Cards.") In other words, she gets right to the center of Oprah's search for hidden mystical meanings. Oprah says she reads Northrup's menopause book "just like it's the Bible. It's the book next to my bed. I read the Bible. I read that book." (Disclosure: NEWSWEEK correspondent Pat Wingert, who worked on this article, and contributor Barbara Kantrowitz are coauthors of a book on menopause.)

Oprah turned to Northrup for advice in 2007, when, as she put it, she "blew out" her thyroid after a stressful season of work and travel. She felt sick and drained and she gained weight. She asked the doctor to come on the show to explain what was going on. "When I called her to talk about this whole thyroid issue," Oprah told the audience, "she always connects the mind, the body and the spirit."

Thyroid dysfunction, which affects millions of Americans (mostly women), occurs when the thyroid gland located in the neck produces too much or too little thyroid hormone. Too much (hyperthyroidism) and the metabolism races, sometimes causing anxiety and weight loss. Too little (hypothyroidism) and it slows, which, if severe, can lead to depression and weight gain. Many things can trigger the disease, especially autoimmune disorders.

But Northrup believes thyroid problems can also be the result of something else. As she explains in her book, "in many women, thyroid dysfunction develops because of an energy blockage in the throat region, the result of a lifetime of 'swallowing' words one is aching to say."

On the show, she told Oprah that "your body gives you signals: 'Hey, you've been putting too much stuff under the carpet ...' "

Oprah : So your body ... is only manifesting what's really going on with your spirit?