The mechanism by which vaccines provide immunity is through the introduction of an antigen into the body, prompting an immune response without causing the disease. Vaccines can contain inactivated pathogens, weakened pathogens, or pieces of the target microorganism (like proteins). Upon exposure, the immune system produces specific antibodies and activates memory cells that "remember" the pathogen. As a result, if the individual encounters the actual disease later, their immune system can respond more rapidly and effectively, preventing illness. This strategy is fundamental in infectious disease prevention.