Depression clearly has a harmful effect on physical health, although the biological reasons for the link between body and mind are unclear. It may be that depression affects health because people develop a fatalistic attitude and stop taking care of themselves.
Whatever the reasons, over the past 20 years, it has become evident that a depressed mood after a heart attack is much more than an "understandable emotional reaction" to a stressful, life-changing event —it is profoundly dangerous, raising a person's chances of having a second, fatal heart attack. In fact, one recent study found that people with depression who suffer a heart attack are at increased risk for complications while still in the hospital.
More recently, researchers have studied the flip side of the equation —the question of whether someone who is depressed is at increased risk for developing coronary heart disease (CHD) down the line. Indeed, prospective studies show that people who had no CHD but were depressed when the studies began were more likely to develop or die of heart disease. In addition, a Norwegian study found that people with significant depression had a higher risk of dying of most major causes of death, even after adjusting for age, medical conditions, and physical complaints when the study began.
Depression also aggravates chronic illnesses such as diabetes, arthritis, back problems, and asthma, leading to more work absences, disability, and doctor visits.