Vaccines work by introducing a weakened or inactive form of a 'foe' microorganism (or parts of it) into the body. This exposure doesn't cause the disease but is enough for the immune system to recognize it as a threat. The immune system then produces antibodies and memory cells specific to that microorganism. If the actual 'foe' microorganism later enters the body, the immune system is already primed to quickly identify and neutralize it, preventing the disease from developing or significantly reducing its severity.