The vaccine is a weakened piece of a virus or bacteria. When it is placed in the body, the body begins to recognize this as something bad. The body recognizes it and attacks it. The vaccine provides a system for the body to recognize things that don't belong there, so an immune response is much faster. Unfortunately, sometimes during this early stage of life, disabilities such as developmental delays, cerebral palsy, autism and other disorders are initially diagnosed.
Some parents suspect vaccines are the cause.
The perception that vaccines cause these disorders is the result of coincidence, not cause and effect, says Freed. Furthermore, vaccines are tested extensively before they come out on the market, he says. Vaccines are safe. They undergo rigorous testing before they're ever opposed for use or recommended for use in this country. Even after they come on the market they're also tested.