A vaccine is a biological preparation that improves immunity to a particular disease. A vac‐ cine typically contains an agent that resembles a disease-causing microorganism, and is of‐ ten 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 foreign, destroy it, and "remember" it, so that the immune system can more easily recognize and de‐ stroy any of these microorganisms that it later encounters.