Myth 5: My baby might get the disease it's supposed to prevent.

That's true of the scariest diseases doctors vaccinate against, such as polio, which was once made with live weakened polio virus. Until this type of vaccine was phased out, around 1994, a tiny fraction of people  -- one in 2.4 million  -- contracted polio from the vaccine itself. But since then, children in the U.S. have received polio vaccine made from killed virus, so there's no risk of contracting the disease from the shot. A few vaccines that are on the schedule do, however, contain live weakened virus to provoke an immune response. These include the MMR and chicken pox immunizations. "These vaccines have the potential to cause some mild illness  -- a little fever and rash," explains Dr. Edwards. "But the illness is much less severe than if a child naturally contracted measles or chicken pox."

Myth 6: Vaccines can contain preservatives that are dangerous.

Until recently, many vaccine concerns centered on the safety of thimerosal, a compound that prevents the vaccine from being contaminated by bacteria and contains a form of mercury called ethylmercury. Mercury in large quantities is known to be harmful to a child's developing brain. Worries about thimerosal's effect on children prompted its removal from nearly all childhood vaccines in 1999. (Thimerosal is still present in some flu vaccine  -- though you can ask your doctor for a thimerosal-free shot.)

Yet it's become clearer since then that ethylmercury does not pose the same health hazard as its cousin, methylmercury, a metal found in the environment that's known to accumulate in the body and cause harm to developing children. "The body is able to eliminate ethylmercury much more quickly than it can eliminate methylmercury," says Dr. Offit.

University of Rochester researchers confirmed that when they compared mercury concentrations in the urine, blood, and stools of children who got vaccines containing thimerosal with those of kids who received only thimerosal-free vaccines. All the children had mercury levels well below the EPA's most stringent public safety limits.

Even if your baby received a vaccine that contained thimerosal, the overwhelming majority of data support a lack of association between the substance and neurological problems, says Margaret Rennels, M.D., the chair of the committee on infectious diseases of the AAP, who points out that children are exposed to mercury from many environmental sources. "The reality that a lot of people seem to miss is that the largest source of organic mercury is the environment: the air we breathe, the water we drink, and the fish we eat. That's due to the burning of coal," she says. You can lessen your child's mercury exposure by limiting the amount of fish she eats. The Food and Drug Administration says that it's safe for young children to eat albacore tuna once a week and fish that are lower in mercury (such as "chunk light" tuna, pollack, salmon, and catfish) twice a week. (Shark, swordfish, king mackerel, and tilefish, which have high mercury levels, are off the menu.)

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