It was a dangerous environment, a heck of a thing to survive. At the center of our defenses was our immune system, our most elegant defense. In this way, think of the brain — the organ that helps us develop habits and customs — as another facet of the immune system. We started washing our hands and took care to avoid certain foods that experience showed could be dangerous or deadly. The immune system responds to our environment.

Source: New York Times March 12, 2019 06:00 UTC