The actual "disease" here is affluenza, or perhaps it's anxiety that overprotective mothers project onto their children. I grew up in a small town, had pets, played in the dirt every day. Nut allergies were unheard of. It's also very interesting that farmers and dirt poor people in 3d world countries don't get these allergies.
I'm not saying you're wrong; I'm sure environment plays a part. But have you considered that one of the reasons why these things were "unheard of" until recently is that advances in communications have made it much easier for news of rarities to be widely disseminated?
The same kind of argument applies to cases of botulism from home canning. Prior to the 20th Century, if someone died from botulism due to home canning, it happened on a farm in the middle of nowhere and didn't have much effect outside of a family and some neighbors. It just wasn't on the radar unless you personally knew people who died that way. Even a doctor might only see a case once every 10 years, so it's not a big deal, right? Plenty of other things to worry about.
But when health records started being compiled for millions of people, it stood out as a problem. The government started programs to educate farmers and gardeners about proper canning methods. It wasn't that suddenly all the home canners got lazy, it was that information networks brought a relatively rare but deadly issue to light, and so we did something about it.
And look, there ARE more cases of allergies in cities. There's millions more people in cities than not, after all.