“If we look at a map of the world today, one of the striking observations is that illnesses like Crohn’s disease are common in more developed countries and rare in less developed ones. The hygiene hypothesis accounts for this uneven distribution by suggesting that less childhood exposure to bacteria and parasites in affluent societies like the United States and Europe actually increases susceptibility to disease by suppressing the natural development of the immune system. This concept has also been linked to the rise of many of our chronic ailments: the obesity epidemic, deadly disorders like metabolic syndrome and heart disease, psychiatric conditions like depression, poorly understood afflictions like autism, and even some forms of cancer—and clinical studies have shown significant disturbances in the microbiome in all of them. We spend huge amounts of time making sure we’re clean—scrubbing ourselves with harsh soaps, sanitizing our hands and environment with chemicals, and eliminating any trace of dirt from our homes and lives—but since the evidence suggests that germs may actually be essential for our well-being, it may be time to rethink our approach to cleanliness and hygiene.”