It requires exposure to a range of foods, bacteria, and even parasitic worms in order to develop its ability to mount an immune response to real threats (such as the bacterium that causes strep throat) while ignoring nonthreats (such as peanut proteins). Vaccination uses the same logic. Childhood vaccines make us healthier not by reducing threats in the world (“Ban germs in schools!”) but by exposing children to those threats in small doses, thereby giving children’s immune systems the opportunity to learn how to fend off similar threats in the future.
