The purpose of vaccines is to help the body build resistance, or immunity, to certain diseases. Germs in vaccines are killed or weakened so they're no longer harmful, but their presence signals the body to make special proteins (antibodies) to fight them.

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Today there are many more vaccines than there were 50 years ago, and more tools and knowledge to make them. But there are also more and different types of germs.

Effective vaccines still aren't available for some of the most deadly diseases, like AIDS, malaria, and tuberculosis.

Although the rate of vaccine-preventable diseases has dropped to low levels in the United States, the viruses and bacteria that cause them still exist.

Vaccines prevent disease in people who receive them and protect those who come into contact with unvaccinated individuals.

Several years ago, there were concerns about the presence of a mercury-based preservative, thimerosal, in common childhood vaccines. The FDA found no evidence of harm caused by doses of thimerosal in vaccines, except for minor local reactions.

However, Public Health Service agencies, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal levels in vaccines should be reduced or eliminated as a precaution.

Today, most vaccines contain no mercury. Some flu vaccines are the exception, but mercury-free flu shots are now available. Be sure to insist on a thimerosal-free vaccine, especially for children.

For more details, visit www.cdc.gov