We ve all heard the mantra exercise for good health. In fact exercise or lack of it may be the most important factor in avoiding or surviving and recovering from the top three killers across developed countries--heart disease cancer and stroke. But few of us understand exactly how different forms of exercise work--physiologically speaking--to keep us healthy and prevent or treat disease. Here two nationally known exercise experts lead a stellar team explaining in reader-friendly terms what exercise does to our bodies and how it spurs beneficial biological actions.