How do vaccines teach your body to fight without making you sick?

The answer

Vaccines introduce a harmless component of a pathogen - a weakened form, an inactivated version, or just a protein fragment - to your immune system. Your body mounts an immune response, producing antibodies and training memory B and T cells to recognize the invader. If you later encounter the real pathogen, these memory cells activate rapidly, neutralizing the threat before it can cause serious illness. This 'dress rehearsal' approach has eradicated smallpox and nearly eliminated polio.