About 100 million Americans live with some form of chronic pain—more than the combined number who suffer from diabetes, heart disease, and cancer. But chronic pain has always been a mystery. It often returns at the slightest provocation, even when doctors can't find anything wrong. Oddly enough, whether the pain is physical or emotional, traumatic or slight, our brains register all pain as the same thing, and these signals can keep firing in the nervous system for months, even years.