Vaccination

Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulating the body's adaptive immunity, they help prevent sickness from an infectious disease.

When a sufficiently large percentage of a population has been vaccinated, herd immunity results.
Herd immunity protects those who may be immunocompromised and cannot get a vaccine because even a weakened version would harm them.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY 2.0

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