From the article:
... researchers are now digging deeper into the mechanisms that underlie the benefits of physical activity. They are finding that exercise is both powerful and wide-reaching, affecting not just muscles and the cardiovascular system, but almost every part of the body, from the immune system to the brain to the energy systems within individual cells. And as scientists understand more precisely which levers exercise pulls to improve our health, clinicians are on the verge of being able to change their practice. The goal is to think of exercise as a medicine — a therapy that they can prescribe in specific doses for specific needs.