How does a vaccine work?

A vaccine works by teaching the immune system to recognise a particular disease-causing germ without the person having to suffer the illness. It introduces a harmless form of the germ or a part of it, such as a weakened or inactivated virus, a fragment of its surface, or instructions for the body to make such a fragment, as with mRNA vaccines. The immune system responds as if facing a real infection, producing protective antibodies and memory cells that remember the germ. If the vaccinated person later encounters the actual pathogen, their immune system recognises it immediately and can fight it off quickly, often before any symptoms develop. When a large enough share of a population is vaccinated, it also provides herd immunity, protecting those who cannot be vaccinated by making outbreaks far less likely. Vaccines have saved countless lives and eradicated smallpox.