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 micro-organism 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 keep a record of it, so that the immune system can more easily recognize and destroy any of these micro-organisms that it later encounters. Vaccines can be
prophylactic (example: to prevent or ameliorate the effects of a future
infection by any natural or "wild"
pathogen), or
therapeutic (e.g.,
vaccines against cancer are also being investigated).