For close to a century now, diseases of the heart or blood vessels have been the top killers of men and women in the United States. In 2007 alone, more than 615,000 people died from heart disease (the single-leading cause of death overall), and more than 135,000 from stroke (the third-leading cause). That’s compared with 560,000 deaths from all forms of cancer.

Heart disease and stroke are forms of cardiovascular disease, or CVD. By the late 1940s, so many Americans were dying from CVD that scientists called it an epidemic. Scientists uncovered the causes of CVD and learned how to stem the epidemic through a famous piece of scientific detective work called the Framingham Heart Study. The story begins in 1948. Death rates for CVD had been increasing steadily since the beginning of the century, but little was known about the root causes. A group of researchers in the town of Framingham, Massachusetts, embarked on an ambitious project to determine the causes of CVD and stop the epidemic.

The researchers recruited 5,209 men and women from Framingham, gave them extensive physical examinations, and interviewed them about their lifestyles. Since 1948, the subjects of the Framingham Heart Study have returned every two years for further exams and lab tests. In 1971, the study enrolled a second generation—about 5,000 of the original participants’ adult children and their spouses. Then, in 2002, the grandchildren of the original participants were added.

Doing this type of “longitudinal” study—a scientific study that follows the same subjects for a long time—allowed the researchers to find common patterns related to CVD that emerged over time. Their work paid off. In 1960, the researchers found that cigarette smoking increased the risk of heart disease, and in the seven years that followed, high blood pressure, high cholesterol, and physical inactivity were likewise linked. Over time, the researchers identified all the major CVD risk factors, which include the four just listed, as well as two more: obesity and diabetes. Much of what we now know about heart disease comes from this study.

Today, this knowledge is being used to design everything from healthier school lunches to healthier communities. While expanding suburbs led to less physical activity and greater obesity among populations—nearly every errand required a car—some communities have begun to fight back. And where sprawl has been curbed, the rates of obesity and chronic illness have started to fall. To support healthier communities, public health officials recommend a blend of stores and services within residential areas, and routes through neighborhoods that promote walking, biking, and heart-healthy lifestyles.

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