Every year, chronic diseases claim the lives of more than 1.7 million Americans. Yet many of these deaths can be prevented. The major chronic disease killers—heart disease, cancer, stroke, chronic obstructive pulmonary disease, and diabetes—are often directly related to how individuals behave in their daily lives. In other words, you can do plenty to prevent them from shortening your life. But first, you need to know something about them.

Cardiovascular diseases are the nation’s leading killers. Diseases of the heart and blood vessels claim nearly 1 million lives annually. Approximately one-third of those people die prematurely—younger than their average life expectancy. Among the most common cardiovascular diseases are coronary artery disease, the leading cause of death; stroke, the third leading cause of death; and hypertension (high blood pressure), which contributes to death from both heart attack and stroke.

Coronary artery disease and most strokes are largely the result of atherosclerosis, a buildup of fat, cholesterol, and other substances on the inside of artery walls. As the walls become lined with layers of these deposits, known as plaque, the arteries narrow, reducing the flow of blood—and oxygen—to the body. When atherosclerosis affects the coronary arteries, which supply blood to the heart, the resulting oxygen deficiency can produce the chest pain known as angina pectoris. If blood flow is severely reduced or blocked, perhaps by a blood clot, the result is a heart attack, in which a portion of the heart muscle literally starves to death. Likewise, a blockage of the carotid arteries, which supply blood to the brain, can lead to stroke—the oxygen-starvation death of an area of the brain.

Although heart disease is the nation’s number one killer, more people fear the number two killer—cancer An estimated 8 million Americans are alive today who have cancer or are cancer survivors, but the disease remains deadly for many. It killed an estimated 564,800 Americans in 1998. Of the more than 100 types of cancer, lung, breast, and colorectal cancers are the deadliest for women. For men, lung, prostate, and colorectal cancers are the deadliest.

Chronic bronchitis may not sound that dangerous, but along with emphysema, it is the fourth leading cause of death in the United States. Collectively known as chronic obstructive pulmonary disease (COPD), the conditions block the flow of air out of the bronchial passages, affecting the body’s ability to get oxygen. In chronic bronchitis, the blockage is caused by persistent inflammation of the lining of the bronchial passages; in emphysema, it’s caused by enlargement of the air sacs in the lungs. COPD killed an estimated 106,146 Americans in 1996. And the incidence of the disease has increased 60 percent since 1982.

Diabetes is the seventh leading cause of death, according the National Center for Health Statistics. It directly caused 61,559 deaths in 1996 and contributed to nearly three times that many. Diabetes is a breakdown of the body’s ability to use the hormone insulin, which enables cells to absorb glucose (blood sugar) for use as energy. In type 1 diabetes, the body virtually stops producing insulin. In type 2 diabetes, the more common form, the cells become resistant to the effects of insulin. In either case, high levels of glucose in the blood result.

High blood glucose levels cause the kidneys to work overtime. And the shortage of sugar causes the body’s cells to use protein and fat for energy. The breakdown of these fats releases toxic acids called ketones, which can poison the body. All in all, diabetes hastens wear and tear on many crucial bodily functions. Left unchecked, diabetes can lead to dangerous complications such as heart disease, stroke, circulation problems, and kidney failure.