Vaccines work by introducing a harmless part of a virus or bacteria (like proteins or inactivated versions) into the body. This triggers the immune system to recognize the pathogen and create **antibodies** without causing illness. Once vaccinated, if the person encounters the actual pathogen in the future, their immune system can respond quickly and effectively, preventing infection or reducing its severity. Vaccines also promote **herd immunity**, protecting those who cannot be vaccinated by reducing the overall spread of disease. They are crucial in preventing infectious diseases, reducing outbreaks, and promoting public health.