• Chris Conway, 54, is thin, eats a healthy diet, takes a baby aspirin every day, and exercises regularly.

• Howard Wainer, 66, has diabetes. Until recently, his blood pressure and blood sugar were too high.

• Naomi Atrubin, 79, has already had two heart attacks.

So who’s at risk? Surprise…it’s all three of them!

Wainer and Atrubin have obvious risk factors, but Conway has to contend with family history—his father had a heart attack in his mid-40s, and died of one at 66. All these people, however, share a common concern about their health: about 1.1 million Americans will suffer a heart attack this year, and some 500,000 will not survive it.

Despite the risks, most people don’t understand what causes a heart attack. The common view is that it’s simply a plumbing problem—cholesterol builds up, clogging arteries like sludge in a pipe. When an artery supplying blood to the heart becomes completely obstructed, portions of the heart, deprived of oxygen, die. The result is a heart attack, right?

Not quite, say heart experts. Heart disease involves the gradual buildup of plaque. And plaque is like a pus-filled pimple that grows within the walls of arteries. If one of those lesions pops open, a blood clot forms over the spot to seal it and the clot blocks the artery. Other things can stop your heart, but that’s what causes a heart attack.

The bigger issue is how to stop it from happening. There’s no way to predict where an artery-blocking clot will originate, so prying open a section of an artery with a stent will not necessarily prevent a heart attack. Stents relieve chest pain, but people who have no symptoms—such as Howard Wainer—are better off adhering to tried-and-true measures to slow plaque growth and prevent the lesions from bursting.

Those measures, says Peter Libby, M.D., chief of cardiovascular medicine at Brigham and Women’s Hospital in Boston, “are things no one wants to hear: keep your weight down, make physical activity a part of your life, stop smoking if you smoke.” And, of course, keep your blood pressure and cholesterol under control, taking medications if necessary.

Few people are following that advice. Twenty-five percent of Americans over age 50 have at least two risk factors, such as high blood-pressure or cholesterol levels, or an elevated blood-sugar level. Only 10 percent of Americans have every risk factor under control.