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A vaccine is a biological preparation that provides active acquired 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, its toxins, or one of its surface proteins. The agent stimulates the body's immune system to
recognize the agent as a threat, destroy it, and to further recognize
and destroy any of the microorganisms associated with that agent that it may encounter in the future. Vaccines can be prophylactic (to
prevent or ameliorate the effects of a future infection by a natural or "wild" pathogen), or therapeutic (e.g., vaccines against cancer, which
are being investigated).