The idea that allergies somehow are caused by our sterile lifestyle is suspicious at best. It's might just be an appeal to nature - "all natural ingredients" fallacy.
I claim that an individual has never before - in the history of mankind - been exposed to so many and different microorganisms as we are now.
Comparing my daughter to my granddad, born 99 years before her, she probably meets as many "new" people per week - if not day, as he did in a whole year when growing up in a reasonably average community not far from a large city.
Me and my wife works in large organisations, and it's representatives are visiting or are visited by hundreds of people from around the world - every week, as do many - most in fact - of the other parents in her school of about 1000 students.
I have lunch in a canteen where approximately 3000 people eat every day, and we all are slopping around in the food warmers and salad bar sharing the utensils and pawing the bread with our halfheartedly washed hands.
On the way to our offices and schools we commute with thousands and thousands of people, all sniffling and coughing it seems.
The interconnectedness of it all is just - mind boggling. We are never more than 2 steps away from someone that were on the other side of the world, yesterday. Hopefully not in an Ebola zone.
A hypothetical "natural" state, where you sleep in a stack of hay with your pigs, and meet one "outsider" twice a a year doesn't seem to tax the immune system even a fraction of what we have to deal with today. Even it's a lot of microorganisms, it's yours (and the pigs) - unless some pigeons shit in your haystack/bed.