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Myth 3: Now that major illnesses have largely disappeared, we really don't need vaccines anymore.

Don't bet on it. Despite our relatively high vaccination rates in the U.S., many American communities still have outbreaks of diseases like measles and pertussis, a respiratory illness characterized by spasms of coughing that can last for weeks or even months. In 2003, 13 children died of the infection.

Unvaccinated children can also spread infection to vulnerable family members. "Those children are more likely to give a disease to those who can't fight it off, such as a six-month-old or a grandparent living at home," says Dr. Saari. The incidence of whooping cough has been increasing since 1980, and the Centers for Disease Control and Prevention recently recommended a pertussis booster shot for 11-year-olds because the risk of passing the disease to a vulnerable relative is so high.

What's more, diseases are spread by people from foreign countries who travel here. "Air travel has extended the range of diseases from countries where people aren't immunized," says Dr. Saari. "We're no more than one airplane ride from being exposed to many diseases."

Myth 4: Vaccines cause autism and other disorders.

Concerns about a link between a combination vaccine for measles, mumps, and rubella  -- called the MMR vaccine  -- and the developmental disorder autism got kicked up by a case report from England seven years ago. But it has been roundly discredited. The notion has persisted because autism tends to emerge around the time that the vaccine is given  -- when a child is a year old. Experts stress, however, that this does not mean the vaccine caused the problem. "Not only is there no evidence that it causes autism, there's evidence that it doesn't cause autism," Dr. Offit says. "In fact, there have been 14 studies that show your risk of getting autism isn't any different if you got the MMR vaccine or if you didn't."

The Institute of Medicine backed up that conclusion in a report issued last summer. Worries linger, Dr. Offit adds, because "it's hard to unring the bell. People reasonably assume that if there is nothing to it, why was there so much smoke?" Parents have expressed similar fears about vaccines and the incidence of sudden infant death syndrome (SIDS). "Numerous vaccines are given to little babies over that first year, just when a lot of developmental changes are occurring," says Dr. Edwards. "If something happens around the time a vaccine is given, it's easy to think the vaccine caused it."