CHRONIC PAIN wrecks the lives of more than 600m people around the world. One in five adults struggles with it. What causes it, however, is often a mystery. The prevailing hypothesis is that, after an injury, something goes wrong with the body’s pain receptors and the alarm system that reports injuries to the brain keeps ringing long after the body has finished healing. Even when there is a clear underlying cause, such as arthritis or nerve damage, it is not known exactly how those conditions produce pain. Treatments, therefore, often bring little relief.