Chronic pain is one of medicine's oldest puzzles and most stubborn problems. About one in five people worldwide live with it, and don't really know why. Dr. Sanjay Gupta says it doesn't have to be that way. A neurosurgeon, CNN's Chief Medical Correspondent, and host of the podcast Chasing Life, Gupta turns his attention to pain why we experience it, what really helps, and how people can take back control. His new book is called It Doesn't Have to Hurt: Your Smart Guide to a Pain-Free Life.