Vaccination is a method of inducing immunity to infectious disease due to bacteria or viruses. Based on the knowledge that second attacks of diseases such as smallpox were uncommon, early methods of protection consisted in inducing immunity by deliberate inoculation of material from a mild case. Starting from the observation that farm workers who had accidentally acquired cowpox by milking infected cows were resistant to smallpox, Edward Jenner in the 1790s inoculated cowpox material into nonimmune persons who then showed resistance to smallpox.
Pasteur extended this work to experimental chicken cholera, human anthrax, and rabies. The term vaccination became general for all methods of inducing immunity by inoculation of products of the infectious organism. Antitoxins were soon developed in which specific immunity to disease toxins were induced.
Vaccination leads to the formation of antibodies and the ability to produce large quantities at a later date; this gives protection equivalent to that induced by an attack of the disease. It is occasionally followed by a reaction resembling a mild form of the disease, but rarely by the serious manifestations.