A vaccine is a biological preparation which improves immunity to a particular disease. A vaccine contains an agent that resembles a disease-causing microorganism and it is usually made from weakened (attenuated) 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 foreign, destroy it and remember it so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.