How does vaccination work?

Vaccines contain killed or modified live forms of viruses or bacteria. They stimulate production of protective antibodies in immunocompetent animals that neutralize the natural virus or bacteria if the animal is later exposed. Although vaccines provide protection against infectious disease, they do not treat or cure existing diseases. Some vaccines contain combinations of viruses or bacteria that immunize against several diseases, minimizing inconvenience to the owner and discomfort for the pet.