A biological preparation containing an agent that resembles a disease-causing microorganism, often a weakened or killed form of the microbe, its toxins, or one of its surface glycoproteins combined with an adjuvant (such as alum, an aluminum salt) which contributes to the immune response. Vaccines act by stimulating the body’s immune system to recognize the agent as a threat, to destroy it, and to recognize it in the future, providing an acquired immunity to that infectious disease. (Synonym: immunization).