Preparation containing either killed or weakened live microorganisms or their toxins, introduced by mouth, by injection, or by nasal spray to stimulate production of antibodies against an infectious agent. This confers immunity to that agent, since the B lymphocytes remain sensitized to it and respond to later infection by producing more antibodies. The first vaccine, against smallpox, was introduced by Edward Jenner in 1798. Vaccines have been developed against diseases caused by bacteria (e.g., typhoid, whooping cough, tuberculosis) and by viruses (e.g., measles, influenza, rabies, poliomyelitis). Effectiveness varies, and a small percentage of people have adverse reactions. Those with immunodeficiency disorders should not receive live vaccines.

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