Immunizations, also called vaccinations, are given to protect people from becoming infected with certain diseases. Fragments of a disease organism or small amounts of a weakened disease organism are introduced into the body, usually by injection, which triggers the development of antibodies that can then recognize and destroy these disease organisms if later exposed to them.
Some immunizations are given only once, while others require several doses over time.
Sometimes an immunization does not completely prevent the disease, but it will significantly reduce its severity.
WebMD Medical Reference from Healthwise