Answer:
immunisations can help protect against certain diseases by imitating an infection. This type of imitation infection, helps teach the immune system how to fight off a future infection. 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.
Once the vaccinated body is left with a supply of T-lymphocytes and B-lymphocytes that will remember how to fight that disease. However, it typically takes a few weeks for the body to produce T-lymphocytes and B-lymphocytes after vaccination. Therefore, it is possible that a person infected with a disease just before or just after vaccination could develop symptoms and get that disease, because the vaccine has not had enough time to provide protection. While vaccines are the safest way to protect a person from a disease, no vaccine is perfect. It is possible to get a disease even when vaccinated, but the person is less likely to become seriously ill.