1. Mouth
Calcified dental plaque from ancient skulls tell us that mouth microbes became less diverse as we switched from varied hunter-gatherer diets to carb-heavy agricultural ones—and the bugs that thrive on all this sugar cause gum disease and cavities. Mouthwash, which wipes out beneficial and halitosis-causing bugs alike, could do more harm than good.
Info from
http://www.motherjones.com/environment/2013/04/bacteria-in-human-body