From the Latin word vacca, meaning “cow,” a vaccine provides immunity from a specific infectious disease by appearing like a disease-causing microorganism, stimulating the body's immune system to build up immunity against the disease. Dr. Edward Jenner discovered that he could prevent the deadly disease smallpox if he purposely injected his patients with cowpox, which was a much milder disease. As the body built up a defense against the cowpox, it simultaneously created an immunity against smallpox. Today, similar inoculations are called vaccines because they began with Dr. Jenner's experiments with cowpox.