For more than 50 years, immunization had saved more billions of lives and prevented countless illnesses and disabilities in the whole world. Vaccine-preventable diseases, such as measles, mumps, and influenza are still a threat. They continue to infect our children, resulting in hospitalizations and deaths every year. A vaccine is a biological preparation that provides active acquired immunity to a particular disease. It typically contains an agent that resembles a disease-causing microorganism 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, recognize and destroy any of these microorganisms that it later encounters.