Vaccines Are Essentially Dead Pathogens
Throughout history, there have been individuals who recognized that after they survived a disease, such as the Black Death, they did not become infected again. In other words, they had become immune because their bodies had produced antibodies that would attack and kill the specific pathogen that caused that disease.
Vaccines build on this idea of creating immunity to a disease by injecting a dead pathogen into the person being immunized. The person’s body may temporarily react as if he or she was becoming sick, but this is actually the process of antibodies being formed that will attack and kill any live version of that pathogen. They will not become infected if they come into contact with that pathogen.