Once we stopped being regularly exposed to pathogens associated with raw unrefrigerated meat, we lost our immunity.

Think of it like traveling to a country with low water quality standards. If you drink the water, you get cramps and diarrhea. The locals have immunity and their babies have immunity because antibodies to the pathogens are passed in mother’s milk. That gives the babies enough immunity to avoid illness when they start drinking the water. Frequent subsequent exposure to the pathogens maintains their immunity.

But if a person with immunity moves to an area with higher quality water and then moves back after many years, they can experience intestinal distress from the water because they lost their immunity.