I'm convinced that in my family it was all to do with our upbringing! We lived in the middle of nowhere, people still had outside toilets (yes, even in the 70's) and we picked fresh veg out of the garden and ate them raw with mud on them. We played in ditches, up trees, made potions with mud, ashes, flower petals, dared each other to eat grass, weeds, absolutely anything. People kept chickens so we were forever collecting eggs, no rubber gloves or disinfectant in sight, we weren't even brought up to wash our hands (yuk!!)
Yet I truly believe we were rarely ill as children. Yes, we had all the usual childhood illnesses, yet I rarely even remember colds. We never had a day ill in bed, and I really do believe we had brilliant immune systems. Is that why, then, my own son obviously has an over active immune system (though I did bring him up to wash his hands I'm glad to say)? He was rarely ill before this. He was a stranger to our GP. He had only ever seen him a handful of times in his life. Since the diabetes diagnosis though, he went on to have cold after cold last year, 8 in total - it was shocking. All very weird, yet interesting.
I should have been a scientist! I am intruiged by all this. One day we will find out and say 'Oh yes......!'