How do vaccines train the immune system?

Vaccines work by showing the immune system a harmless version or fragment of a germ — such as a weakened virus, an inactivated one, a protein, or genetic instructions to make a protein — so the body learns to recognise it without getting sick. In response, the immune system produces antibodies and memory cells tuned to that specific threat. If the real germ later invades, these memory cells recognise it and mount a fast, powerful defence before the infection can take hold. This is why vaccination provides protection that can last years or a lifetime. Widespread vaccination also creates herd immunity, protecting those who cannot be vaccinated by slowing the spread of disease.