Commonly Administered Vaccines, continued

The vaccinations given in childhood against measles, mumps, whooping cough, and polio are weakened or killed forms of the viruses or bacteria causing these diseases. In live attenuated vaccines, the microorganism in the vaccine is too weak to make the child sick but is strong enough to stimulate the immune system to produce memory cells to the virus or bacteria. When the child is later exposed to the wild virus or bacteria, these memory cells are activated and attack the wild virus or bacteria very quickly and prevent the child from becoming sick.