In this episode of "THE BODY," we explore one of mankind's oldest desires -- finding the secret to a long, healthy life. Cutting-edge science has revealed new images of the remarkable gigantic network that stretches across the human body. And playing a key role are the "message molecules" released by our organs. Now, the power of these message molecules is about to significantly change the way we treat serious illnesses such as cancer and heart disease and help determine how well -- and how long -- we live.