A vaccine is a biological preparation that provides active acquired immunity by training the immune system to recognize and destroy specific pathogens, typically using weakened or killed forms of the microbe or its surface proteins. Vaccines can be prophylactic (preventing future infections) or therapeutic (fighting existing diseases like cancer), and their administration, called vaccination, is the most effective method for preventing infectious diseases. The practice dates back to 16th-century China for smallpox (with hints from the 10th century), was brought to Britain by Lady Mary Wortley Montagu in 1721, and was formalized by Edward Jenner in 1798 using cowpox, with Louis Pasteur expanding the terminology in 1881. Their impact is staggering: vaccines led to the eradication of smallpox and drastically reduced diseases like polio, measles, and tetanus—for example, US measles cases dropped from 763,094 in 1958 to fewer than 150 per year, and the measles vaccine prevents an estimated one million deaths annually. This success depends on herd immunity, where widespread vaccination blocks outbreaks, and the World Health Organization currently lists licensed vaccines for 25 different preventable infections.