A vaccine is a biological preparation that provides immunity to a specific infectious disease. It typically contains a harmless part of the virus or bacteria, such as a weakened or inactivated form, or a piece of its genetic material. When introduced into the body, vaccines stimulate the immune system to recognize and fight the pathogen if encountered in the future. Vaccines are crucial in preventing diseases like measles, polio, and influenza. They help reduce the spread of infections and protect individuals and communities. Widespread vaccination can lead to herd immunity, which protects those who cannot be vaccinated, such as infants or individuals with certain health conditions.