Every year, nearly 800,000 Americans suffer a stroke.And 40 percent end up having a second stroke which can be even more debilitating.But the key to prevention may be very simple.Research found a critical step that many doctors and patients are ignoring.Nick Sparks loves spending time out and about with his wife Liz.But recently walking became difficult for this 74-year old.A trip to get checked at his doctor averted what could have been a devastating diagnosis."All of the stroke doctors had difficulty understanding why I had not had a stroke," said Sparks.The arteries in Nick's legs and leading up to his brain were dangerously narrowing.Doctors performed angioplasty to clear the clogged arteries.It was a close call. The reason? One critical factor is being overlooked."Exercise is the neglected prescription," said Dr. Fadi Nahab, the stroke medical director at Emory University Hospital.Nahab was the lead investigator for the SAMMPRIS Trial.It studied the recovery of patients who had mini-strokes and found those who were physically inactive were six times as likely to have another stroke.Nahab tells patients to walk, and work up to 150 minutes every week of moderate exercise.He recommends a brisk walk or 75 minutes of vigorous exercise such as jogging or running."If you were talking to somebody, you wouldn't be able to say a full sentence without taking a breath," Nahab said.The exercise can be broken up into smaller sessions several times a day.Sparks is exercising daily now for the first time in his life."As I begin to do those little things on the side, they are just as beneficial as going to the gym every day," Sparks said.His doctors said it's a prescription he can't ignore.There's a higher rate of death and disability with recurrent strokes because parts of the brain are already injured.But experts say 80 percent of secondary strokes can be prevented with medication and lifestyle changes, including more physical activity.