The “hygiene hypothesis,” sometimes referred to as the “old friends hypothesis,” is that humans have evolved with certain intestinal bacteria and worms whose presence can “train” the immune system not to overreact against harmless commensal organisms. As people have become more prosperous and are leading “healthier” lifestyles, these organisms have been lost from the intestines, and the immune system is losing its natural balance. This predicts a significant increase in immune diseases, both allergic and autoimmune, which in fact has been observed in the developed world over the last several decades.