Immunization

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Immunization, also called vaccination or inoculation, a method of stimulating resistance in the human body to specific diseases using microorganisms—bacteria or viruses—that have been modified or killed. These treated microorganisms do not cause the disease, but rather trigger the body's immune system to build a defense mechanism that continuously guards against the disease. If a person immunized against a particular disease later comes into contact with the disease-causing agent, the immune system is immediately able to respond defensively.

Scientists have developed two approaches to immunization: active immunization, which provides long-lasting immunity, and passive immunization, which gives temporary immunity. In active immunization, all or part of a disease-causing microorganism or a modified product of that microorganism is injected into the body to make the immune system respond defensively. Passive immunity is accomplished by injecting blood from an actively immunized human being or animal.

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