You probably have a vague sense that exercise is good for you-and you've probably heard that it's "healthy for the heart." But if you're like most people, that's not enough motivation to get you to break a sweat with any regularity. As  I report in the TIME cover story, "The Exercise Cure," only 20% of Americans get the recommended 150 minutes of strength and cardiovascular physical activity per week, more than half of all baby boomers report doing no exercise whatsoever, and 80.2 million Americans over age 6 are entirely inactive.

That's bad news, but emerging evidence shows that there are plenty of compelling reasons to start moving at any age and even if you're ill or pregnant. Indeed, scientists are learning that exercise is, actually, medicine. "There is no pill that comes close to what exercise can do," says Claude Bouchard, director of the human genomics laboratory at Pennington Biomedical Research Center in Louisiana. "And if there was one, it would be extremely expensive."