It is widely assumed that stress makes us more susceptible to disease. It is thought that stress affects our immune system through one of three pathways: through nerve fibers connecting the central nervous system to the immune system, by altering production of hormones, or by adopting behaviors (such as smoking or drinking) to combat stress that suppress the immune system.

There are studies showing impaired immune system functioning among people taking important examinations (Kiecolt-Glaser et al, 1984), people subjected to acute laboratory stressors (Manuck et al, 1991), couples experiencing marital conflict (Kiecolt-Glaser et al, 1987), and people under perceived stress (Jabaaji et al, 1983).

Although these studies show that stress causes immunosuppression, they do not prove that stress makes you more susceptible to disease. Because such studies of stress are done in a relatively short time, and use few indicators of immune system functioning, they cannot give an exact estimate of the body's ability to resist disease (Cohen & Williamson, 1991).

Naturalistic studies of stress also show a relationship between stress and self-reported colds and influenza (for a review, see Cohen & Williamson, 1991). However, these studies do not provide a conclusive link either. The relationship between stress and illness may have been due to another factor not examined in the studies. Another problem is the fact that it is impossible to tell if the people reported feeling sick because they were stressed, or reported feeling stressed because they were feeling ill.

Cohen, Tyrell, and Smith (1991, 1993) have used a technique that allows for the assumption of causality. After completing a stress questionnaire, subjects are exposed to a virus that causes the common cold (administered through nasal drops). Subjects are then quarantined and monitored for approximately seven days. Bodily fluids are examined in order to determine infection, which can identify subjects who have become infected but have yet to show symptoms. A control group was given saline nasal drops instead of the virus. The procedures used in this study eliminate many of the problems inherent in the other research designs.

The results of these studies showed that subjects with higher reported levels of stress were more likely to be infected with the cold virus. However, these results could not be explained by differences in smoking, drinking alcohol, exercise, eating, or sleeping habits. Therefore, even though the results showed a strong link between stress and susceptibility to a cold virus, there was no proof that this finding has a biological or behavioral cause.

• Cohen, S. and Williamson, G.M. (1991). Stress and infectious disease in humans. Psychological Bulletin, v109. pp. 5-24.

• Cohen, S. and Tyrell, D.A.J., and Smith, A.P. (1993). Life events, perceived stress, negative affect and susceptibility to the common cold. Journal of Personality and Social Psychology, v64. pp. 131-40.

• Jabaaji, L., Grosheide, P.M., Heiftink, R.A., Duivenvoorden, H.J., Ballieux, R.E., and Vingergoets, A.J.J.M. (1993). Influence of perceived psychological stress and distress on antibody response to low dose rDNA hepatitis B vaccine. Journal of Psychosomatic Research, v37. pp. 361-9.

• Kiecolt-Glaser, J.K., Garner, W., Speicher, C.E., Penn, G.M., Holliday, J. and Glaser, R. (1987). Psychosocial modifiers of immunocompetence in medical students. Psychosomatic Medicine, v46. pp. 7-14.

• Manuck, S.B., Cohen, S., Rabin, B.S., Muldoon, M., and Bachen, E. (1991). Individual differences in cellular immune response to stress. Psychological Science, v2. pp. 111-5.

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