Why do vaccines make you sick?

Vaccines are not the disease itself. They make the body think that it is being attacked and it responds by making antibodies against that virus or bacteria. You can have some side effects where you may not feel well. These feeling is also a way the body fights diseases off. You know how you feel with cold infecting you. Part of that is the response you body makes when it is infected. A cold, flu, some other infection often have the same feelings (symptoms).