Our immune systems evolved to be active. Just like our muscles and skeletons evolved to be used and stressed, our immune systems evolved to cope with all of those germs in the outside world. We’ve now created environments that are very sterile, that are extremely clean; we have very few pathogens that we have to deal with. And if we do get them, we nuke them with antibiotics. In so doing, we are now affecting how our immune system functions; it’s still there, and it’s primed and ready and waiting to attack all those germs and worms that used to make us sick, but now those pathogens are absent, so it sometimes by chance finds the wrong targets. So that’s the hypothesis for why so many allergies and autoimmune diseases are on the rise — is that our immune systems are essentially not being used properly, and as a result they go into overdrive; they attack ourselves.