Vaccines are medical products that are designed to prevent infectious diseases by stimulating the body's immune system to recognize and fight off harmful pathogens. They work by introducing a small, harmless piece of the pathogen (such as a protein or a weakened version of the virus) into the body, which triggers an immune response that prepares the body to quickly recognize and destroy the actual pathogen if it is encountered in the future. Vaccines have been instrumental in preventing and eradicating many deadly diseases, such as smallpox, polio, and measles, and continue to be one of the most effective public health tools available today.