A vaccine is a biological preparation
that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism
and is often made from weakened or killed forms of the microbe or its toxins. The agent stimulates the body’s immune system to recognize it as foreign, destroy it, and ”remember” it, so that the immune system
can more easily identify and destroy any of these microorganisms that it encounters later. The body’s immune system responds to vaccines as if they contain an actual pathogen
, even though the vaccine itself is not capable of causing disease. Because vaccines are widely used in the United States, many once-common diseases—polio
, diphtheria, whooping cough, mumps
, and certain forms of meningitis
—are now rare or well controlled.
The body’s immune system responds to vaccines as if they contain an actual pathogen, even though the vaccine itself is not capable of causing disease.
Vaccinated people produce antibodies
that neutralize a disease-causing virus or bacterium. They are much less likely to become infected and transmit those germs to others. Even people who have not been vaccinated may be protected by the immunity of the “herd,” because the vaccinated people around them are not getting sick or transmitting the infection. The higher the proportion of vaccinated people in a community, the lower the likelihood that a susceptible person will come into contact with an infectious individual—leading to greater herd immunity.
In the past, thimerosal, a preservative that contains mercury, was used in some vaccines and other products. Use of this product became the subject of controversy, with some arguing that the substance caused autism in children. Extensive, independent research has presented no convincing evidence of harm associated with the low levels of thimerosal present in vaccines. Since 2001, thimerosal has not been routinely used as a preservative in recommended childhood vaccines.
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