I've heard a lot of concerns that monocrop agriculture with pesticide and herbicides especially in American farming practice along with the waves of preservatives and other chemicals added to our food supply for the past few generations may have had an adverse effect on our intestinal microbiomes, contributing to the growth in allergies and chronic illnesses. I don't know how true it is, but despite some of the quacks I see spouting these theories, I consider it plausible for now.