[Mister Da]

I’m guessing 2 things:

Early exposure to a multitude of natural allergens from living a country life.

Consumption of relatively natural, unadulterated foods & drink.

I think science will ultimately find that young children need wide exposure to the natural world, not sheltering from it, in order to build the immunities they need in life. City living is inadequate to fortify a child to live in a dangerous world.

[dirtboy]

I would throw in a third - lots of physical exercise.