Babies are born with some antibodies (infection-fighting substances) in their bloodstream that were passed on to them in the womb from their mother. But as those inherited antibodies decrease in the first year of life, a baby must develop new antibodies and other infection-fighters. When he gets a cold or the flu, for instance, a baby develops antibodies against the illness that protect him from catching the same virus as easily again. However, there are some serious diseases -- like whooping cough or polio -- that you wouldn't want your child to catch just so he can become resistant to them. That's where vaccines come in. Vaccines teach a baby's immune system how to recognize and fight off specific infections. Most vaccines are made from an inactive form of a virus, though a few (i.e., the measles vaccine) are made from a weakened form of a live virus. These vaccines don't cause disease, but can stimulate a baby's body to recognize it and fight it off.
Originally published in American Baby magazine, February 2005. Updated 2009.