I think diet is a big part of it and another part, no idea how big, is everyone constantly using anti-bacterial soaps and cleaners and wipes and purell, etc. I mean, if you want to buy hand soap you have to go out of your way to find one that isn't antibacterial.my best guess is that there are multiple factors causing this - overuse of antibiotics, stronger antibiotics (because other bugs have become resistant to the older drugs), crappy American diet, who knows.