How do vaccines work?

You ask: How vaccine work?

Caveman body have a warrior tribe inside, fighting off tiny invaders that make Caveman sick. Trouble is, first time a new invader come, warriors slow — they learn while Caveman already sick in cave. Vaccine fix this. It show the warriors a dead or weak piece of invader, like showing hunters the paw-print and smell of mammoth before the hunt. No real danger, but warriors study the shape, make spears (antibodies) that fit it, and keep some scouts (memory cells) who remember. Later, real invader sneak in — scouts spot the shape at once, warriors swarm fast, crush it before Caveman even feel sick. This practice-hunt trick save about four million lives every winter. Cave wisdom: vaccine not medicine that fight for Caveman — it a practice hunt, so body already know the enemy face before the real fight come.