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.