Immunizations have had an enormous impact on improving the health of children and adults in the United States. Most people today have not had first-hand exposure to the devastating consequences that vaccine-preventable diseases have on a family or community. While vaccine-preventable diseases are not as common in the U.S. as they are in other communities around the world, cases continue to present. One of the best ways to protect infants, children, and adults against these potentially harmful diseases is through vaccination.
Vaccines reduce the risk of infection by working with the body's natural defenses to help it safely develop immunity to disease. When germs, such as bacteria or viruses, invade the body, they attack and multiply. This invasion is called an infection, and the infection is what causes illness. The immune system then has to fight the infection. Once it fights off the infection, the body is left with a supply of cells that help recognize and fight that disease in the future.
Vaccines help develop immunity by imitating an infection, but this "imitation" infection does not cause illness. It does, however, cause the immune system to develop the same response as it does to a real infection so the body can recognize and fight the vaccine-preventable disease in the future. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever. Such minor symptoms are normal and should be expected as the body builds immunity.