'Old Friends Mechanism'
He proposed that the required microbial exposures are not infectious diseases (colds, flu, measles, norovirus etc) which evolved only over the last 10,000 years, but the microbes we co-evolved with in hunter gatherer times when the human immune system was developing.
Although these microbes are still there, through modern lifestyle changes we have lost our exposure to them
Dietary Influences
eliminating harmful organisms - at the same time we have inadvertently also eliminated Old Friends microbes from our food.
changes in dietary habits
consumption of animal fat versus carbohydrates) have led to changes in types and balance of microbes in our gut.