A vaccine is a medical product designed to stimulate the immune system to develop protection against specific infectious diseases. Vaccines typically contain weakened or inactive forms of pathogens, or their components, to trigger an immune response without causing illness. They play a critical role in public health by preventing outbreaks, reducing mortality, and controlling the spread of contagious diseases. Vaccination programs are key tools in combating illnesses such as influenza, measles, and COVID-19.