(Dhabhar and McEwen, 1999). The relationship between stress and the immune system has been demonstrated in animal models, which have shown that stress makes animals more vulnerable to experimental tumors (Ben-Eliyahu et al., 1991) and infections (Ben-Nathan and Feuerstein, 1990; Ben-Nathan et al., 1991; Bonneau et al., 1991; Friedman et al., 1965; Rasmussen et al., 1957).

Research with humans also has shown that immune function may be altered by affective states and by major and minor acute and chronic stressful life experiences (Biondi, 2001). Chronic stress and a lack of social support, for example, increase the likelihood that a person will develop a cold after being challenged with a standard dose of a rhinovirus (Cohen, 1995). In addition, stress-induced modulation of the immune system has been linked to the expression of inflammatory, infectious, and autoimmune diseases.

Psychological, Social, and Behavioral Factors in Chronic Disease

In the year 2000, roughly 125 million Americans—nearly half of the U.S. population—were living with some type of chronic condition (Partnership for Solutions, 2003). Sedentary lifestyles, poor dietary habits, and the large population of aging baby boomers have all contributed to the rising rates of age- and lifestyle-related chronic medical conditions, such as diabetes, heart disease, and arthritis. The number of cancer patients has grown steadily over the past two decades, and these patients are surviving longer than ever before as a result of improvements in early detection and treatment of the disease. Likewise, the widespread use of potent combination antiretroviral therapy has led to a growing population of people living with HIV infection, who retain a potentially lifelong risk of spreading this infection to others (IOM, 2003b).

These trends have led to recognition that medical students must be educated in the psychological, social, and behavioral factors that can potentially lead to chronic medical conditions and in the interplay between these factors and particular chronic illnesses. For example, it is strongly believed that hostility, chronic stress, depression, social isolation, and increased use of alcohol and tobacco are related to an elevated risk of coronary heart disease (Barefoot et al., 2000; Carroll et al., 1976; Frasure-Smith et al., 1993; Kawachi et al., 1996; Orth-Gomer et al., 1993). Conversely, changing behaviors that may place a person at risk of myocardial infarction, such as hostility and impatience, can reduce the risk of reinfarction in post-myocardial infarction patients (Friedman et al., 1986; Mendes de Leon et al., 1991). Studies have also shown that psychological and social factors influence the development and course of cancer (Everson et al., 1996; Watson et al., 1999).

Other behaviors, such as engaging in risky sexual practices or sharing needles with an HIV-infected partner, significantly affect whether an uninfected patient will contract HIV. Medical students should know which individuals are at greatest risk of becoming infected with HIV and which are most likely to continue to



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