Edward Jenner, an English physician, is credited with inventing the world's first vaccine. He observed that milkmaids who had contracted cowpox seemed to be immune to the much more deadly smallpox. In a famous experiment, he inoculated a young boy with cowpox and later showed he was immune to smallpox. This discovery laid the foundation for modern immunology and the eventually successful global effort to eradicate smallpox, which was officially declared gone in 1980. The word 'vaccine' actually comes from the Latin 'vacca,' meaning cow. Jenner's work has saved hundreds of millions of lives and changed the course of human history by demonstrating that medical science could proactively prevent infectious diseases through immunization.