The immune system is the body’s natural defense against germs. When a germ invades, the immune system sends special cells to fight it off. Sometimes, though, the immune system is not naturally strong enough to prevent a disease from seriously harming someone or even killing them. But there’s a way to give the immune system a boost. It’s known as a vaccine.
Vaccines are substances that prepare the immune system to fight a disease-causing germ or other pathogen by imitating an infection. They trick the immune system into making a “memory” of that germ without ever having to fight the real germ in the first place. Now, when the immune system encounters the real pathogen — whether it’s a virus, bacterium or other microbe — it is ready to attack it. As a result, the vaccinated person doesn’t get sick.