Don't touch that - you don't know where it's been!

Young boy near where I live is highly allergic to peanuts. He has to carry an epipen (I think it's called) around with him at all times just in case he is exposed to the slightest hint of them. So I imagine his parents will be very interested in the recent reports that there is now a way of combating the allergy by giving the sufferer very small regular doses of peanut flour. But this once again raises the question for me of where and when such allergies arose. I'm pretty sure when I was a kid that peanut allergy and dairy product allergy and so on were unheard of. When did it all start, and why? I've mentioned this before, but is it connected to our modern-day obsession with cleanliness? Do we need a certain amount of exposure to muck and dirt when we're little to prepare our bodies for what they may come up against? Is any research being done on this?