When was the last time you saw a child with polio or measles? Vaccines have all but done away with these once-common ailments. Still, that doesn't mean they don't exist. It just means the vaccines are doing their jobs.

Immunizations have cut most vaccine-preventable diseases by more than 99 percent, says the Centers for Disease Control and Prevention (CDC). Yet illnesses that are rare in this country thrive in parts of the world just a plane ride away. Each year, for instance, dozens of measles cases slip into America from abroad, putting at risk those who haven't had their shots.

During a drop in immunization rates a decade ago, a measles outbreak of 55,000 cases across the United States hospitalized 11,000 and killed 125, according to Carden Johnston, M.D., former president of the American Academy of Pediatrics (AAP).

"There was a 25 percent mortality rate from pertussis [whooping cough] before there was a vaccine," says Keith R. Powell, M.D., who serves on the AAP infectious disease committee.

Vaccines protect your community, as well as your children. When more children get their shots, it's less likely that an outbreak can spread. This is sometimes referred to as "community immunity" or "herd immunity," the point at which enough people are vaccinated to protect those who are not.

Severe side effects are very rare, the CDC says. Many studies have tested whether vaccines cause autism or other childhood diseases, says Dr. Powell, and none has found a link.

"I was in grade school in the '50s, and every class had a kid with polio," Dr. Powell recalls. "Now, you just don't see it, and that's because of the vaccine."