Vaccines work by stimulating the immune system to recognize and defend against specific pathogens. They contain weakened or inactivated forms of the pathogen, parts of the pathogen, or modified toxins produced by the pathogen. When a vaccine is administered, the body recognizes these components as foreign and mounts an immune response, producing antibodies and activating immune cells specific to the pathogen. This immune response creates a memory of the pathogen, so if the person is later exposed to the actual infectious pathogen, their immune system can quickly recognize and mount a defense, preventing or reducing the severity of the disease. Vaccines have been instrumental in preventing numerous diseases and have played a crucial role in public health by reducing the spread of infections and saving lives.