Bob Harper was the picture of health and then he had a heart attack. How does that happen?

Biggest Loser host Bob Harper makes his living telling others their lives depend on exercise, weight control and other healthy habits. This week, the 51-year-old fitness guru told fans he is recovering from a heart attack.

How could that happen?

It’s not as unlikely as it may seem. Heart disease is the leading killer of men and women in the United States, and about 735,000 Americans suffer heart attacks each year, according to the Centers for Disease Control and Prevention.

'Biggest Loser' host Bob Harper 'taking it easy' after heart attack

Not all those people have obvious risk factors. But Harper suggested he has at least one: his mother died of a heart attack, NBC’s Today reported. That kind of family history can increase risk, according to the American Heart Association.

And while Harper may seem relatively young, he is in the company of the 3% of U.S. men and 2% of U.S. women who have heart attacks between ages 40 and 59, says the heart association.

Heart attacks become more common after age 60. Other risk factors include high blood pressure, high cholesterol, diabetes and smoking.

And what about exercise? Harper told TMZ he collapsed while working out in a New York City gym. While vigorous exercise can sometimes act as a trigger for a heart attack, it’s less likely to happen in someone who is already fit, according to the American College of Sports Medicine. The overall heart benefits of exercise “far outweigh” any risk, the group says.

“Physical fitness and a heart healthy diet don’t confer immortality,” but do lower risks, says Prediman K. Shah, a cardiologist and professor of medicine at Cedars-Sinai in Los Angeles. Good health habits also help people recover from heart attacks and prevent recurrences, he says.

Shah says many young heart attack patients recall no warning signs and have never been properly screened for the most common underlying cause, coronary artery disease. That’s a build-up of fat, cholesterol and other substances in arteries, and it can happen even in people who look and feel healthy, Shah says. A strong family history – especially heart disease in a father before age 50 or a mother before age 60 – is a good reason to ask your doctor about screening tests, he says.

And everyone should know their blood pressure and cholesterol numbers, as well as the warning signs of heart attack, such as chest pain or pressure and unusual shortness of breath.

As for Harper, he wrote in an Instagram post that he is home after a hospital stay: "I am feeling better. Just taking it easy."