How vaccine work?

Someone ask: How vaccine work?

Vaccine work by showing body small, safe piece of germ or copy of germ, like tribe showing weak practice spear before real hunt. Body see that piece and think, “invader!” Then body make soldiers and memory. Some soldiers fight now, and some become memory guard, waiting in cave for later attack. If real germ come, body remember fast and smash it before big sickness grow. Vaccine not make body sick on purpose; it teach body pattern of enemy, so next fight is quick. Cave wisdom: best battle won when tribe learns enemy face before war begin.