Flu from A to Z

A vaccine is a biological preparation used to stimulate the production of antibodies and provide immunity against one or several diseases. It is prepared from weakened or killed forms of the causative agent of a disease, its products, or a synthetic substitute, treated to act as an antigen without inducing the disease. After treatment with a vaccine, the body's immune system has learnt to recognize the agent as foreign, destroy it, and "remember" it, thus being ready to face future encounters.
Vaccines can be prophylactic (e.g. 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).