When germs enter the body, the immune system recognizes them as foreign substances (antigens). The immune system produces the right antibodies to fight the antigens. Vaccines contain weakened versions of a virus or versions that look like a virus (called antigens). This means the antigens cannot produce the signs or symptoms of the disease, but they do stimulate the immune system to create antibodies. These antibodies help protect you if you are exposed to the virus in the future. Vaccines not only help keep your child healthy, they help all children by stamping out serious childhood diseases. Immunizations are recommended, because they protect against diseases (give immunity) or make a disease less severe if your child does get it. Vaccines protect children from more than a dozen diseases such as measles, polio, tetanus, diphtheria, and pertussis (whooping cough). Most of these diseases are now at their lowest levels in history, thanks to years of immunization. The baby vaccination schedule outlines the immunizations and booster shots needed from birth through age 18, as well as when catch-up immunizations should be given. The baby immunization schedule for a premature infant is the same as for a full-term infant. Many immunizations require more than one dose, given at varying intervals. Although the child does not need to restart the series if a scheduled dose is missed, the immunization should be given as soon as possible.