Vaccination

Vaccination is the deliberate exposure of a person to a weakened, killed, or related but milder form of a disease-causing agent, training the immune system to recognize and fight the real disease without the risk of contracting it directly. Edward Jenner introduced the technique in 1796 using cowpox to protect against smallpox, and it spread and worked for decades on pure empirical observation, well before germ theory of disease or the science of immunology existed to explain why it worked. Once those fields did exist, they gave vaccination its mechanism rather than its origin: the reason a milder or related exposure trains real immunity was only understood long after the practice had already proven itself. Modern vaccines use a much wider range of methods than Jenner's live related virus, but the underlying principle he demonstrated is unchanged.