Well there's that theory that a reduced exposure to infections during the first few years of life causes the immune system to overreact to harmless things. Mode

Well there's that theory that a reduced exposure to infections during the first few years of life causes the immune system to overreact to harmless things. Modern sterile environments are quite the opposite of kids playing in the dirt a century ago. Not sure if that theory still holds though. Then there's the genetic component, if you had allergies before epipens and ambulances it was pretty likely that you just, well, died, the first time you got exposed to the allergen. Today people with dozens of deadly allergies live relatively normal lives, and are able to make those genes propagate further. This goes for most genetic problems really, any ones we can treat will eventually become massively more prevalent than before.