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considerations as whether expert medical advice is freely and conveniently available. Sickness-related behaviour is often more a reflection of psychological factors than physical health.

      To complicate matters further, people’s perception of their own state of health varies according to their mental and emotional state. Anxious or stressed individuals, for example, are more apt to notice and worry about minor symptoms, interpret them as evidence of disease and seek expert help. Someone who has been experiencing lots of stressful life events is more likely to feel unwell and visit their doctor, but this does not necessarily mean that they are actually ill.

      We shall be looking at this issue in more depth in the next chapter. Suffice it here to say that there is a world of difference between believing yourself to be ill, or going to the doctor, and having a clinically verifiable disease. For this reason, research that relies wholly on self-assessments of health or on sickness-related behaviour can be misleading. Such measures often say more about people’s mental state than they do about their true physical health. I should add, however, that the dubious practice of using sickness-related behaviour as an ersatz measure of health is a pervasive problem in medical research and is certainly not unique to work on life events.

      Despite these caveats there is consistent evidence, garnered from thousands of scientific studies, for a connection between life events and subsequent illness. It is now clear that even the mundane hassles of everyday life have an impact on physical health. Indeed, some scientists have argued that because these hassles are such a frequent occurrence their cumulative influence on health may be more pervasive than the effects of rarer, but more traumatic life events.

      The general idea that psychological factors can affect susceptibility to physical illness is amply supported by research on other species. As in so many other respects there is nothing biologically unique about humans. Several decades of experimental work on other species have confirmed that various forms of psychological stress can increase (or, occasionally, decrease) animals’ susceptibility to a wide spectrum of diseases, including bacterial and viral infections, heart disease and cancer.

      For instance, when mice or rats are exposed to stressful situations, such as being physically restrained or subjected to unpleasant electric shocks, they become less resistant to infection with a whole range of bacteria, viruses and parasites including mycobacteria (the type of bacteria responsible for tuberculosis), herpes viruses, influenza viruses, polio viruses and the protozoa which cause toxoplasmosis. In one experiment, for example, frightening mice by exposing them to a cat significantly increased their vulnerability to infection with a parasitic tapeworm. (The cat was prevented from attacking the mice; the sight of it alone was enough to affect them.) Likewise, the social stress of being introduced into an unfamiliar flock makes chickens more susceptible to bacterial infections, while the stress of being transported renders cattle vulnerable to a form of viral pneumonia caused by the reactivation of latent herpes viruses.

      The sheer volume of animal research in this field makes it impossible to describe more than a tiny and rather haphazard selection of examples. And some of the experiments, especially those performed in the dim and distant past, are too grisly and unethical to deserve a mention. We humans are not the only animals whose physical health can be damaged by upsetting events.

      

The mind and the common cold

      The way in which psychological factors can affect our susceptibility to disease is illustrated by research on that most mundane of illnesses, the common cold.

      For centuries it has been widely believed that stress makes us more prone to minor respiratory infections such as colds and ’flu. This has now been confirmed experimentally. It is surprising that until recently much of the scientific evidence regarding the effects of psychological factors on respiratory infections was suggestive rather than conclusive.

      In one study, for example, researchers asked married couples to fill in a questionnaire each day for three months, recording the various stresses and hassles of everyday life together with their state of health. The results showed that respiratory infections tended to be preceded by a greater than average degree of stress. Typically, a few days before the onset of symptoms there would be a rise in the number of unpleasant life events and a drop in the number of desirable events.

      Much firmer evidence came from a pioneering experiment in which psychologist Richard Totman and colleagues infected healthy volunteers with cold-inducing rhinoviruses, having first assessed each individual’s psychological profile. It transpired that personality and previous exposure to stress had a significant bearing on both the risk of infection and the severity of the subsequent cold. Individuals with introverted personalities developed more severe colds, as did those who had experienced certain types of stressful life events.

      The volunteers in this experiment were deliberately infected with viruses in order to avoid a potential ambiguity that had undermined previous research. Critics had pointed out that a correlation between psychological factors and colds could be attributed to varying degrees of exposure to cold viruses, rather than anything to do with biological resistance to infection. Individuals with shy personalities, say, or those who have recently experienced a traumatic life event, might be inclined to stay at home and would therefore have fewer opportunities to catch a cold.

      By exposing all subjects equally to cold viruses Totman’s experiment excluded this possibility. The fact that psychological measures still predicted the clinical outcome implied a more direct link between mental state and disease.

      The technique of deliberately exposing people to bacteria or viruses in order to assess their vulnerability had, incidentally, been used before. In one hair-raising experiment in the early 1970s a group of healthy (and obviously well-motivated) volunteers were exposed to bacteria which cause a plague-like disease, with symptoms including prolonged fever, vomiting, headaches and swollen lymph nodes. Two days before they were infected each subject’s stress level was assessed using standard psychological techniques. Those who registered the highest stress levels went on to have the most severe fevers.

      Further compelling evidence for a connection between psychological stress and colds came a few years ago from a similar experiment. It is worth considering this experiment in detail because it illustrates some important general points.

      Sheldon Cohen and colleagues recruited 420 healthy men and women and installed these worthy volunteers in residential accommodation at the British Medical Research Council’s Common Cold Unit in Salisbury. They then used standard psychological techniques to assess the mental state and stress level of each volunteer. Specifically, the researchers noted the life events that each subject had experienced over the previous year; the extent to which subjects perceived themselves as unable to cope with the demands placed on them by life; and each individual’s current emotional state. The volunteers were then exposed to a standard dose of cold viruses which matched the level of virus exposure one might expect to find in normal life. Each subject was given nasal drops containing one of five viruses capable of producing a common cold.³

      Over the following week the subjects were monitored to see if they had been infected and, if so, whether they then developed clinical symptoms of a cold. Each day a doctor examined them for signs and symptoms of a cold using a standard checklist.⁴ (So this experiment, you will notice, was immune from the criticism that stress might have affected the subjects’ sickness-related behaviour as opposed to their actual health.)

      The results of Cohen’s experiment were clear and compelling. The more psychological stress an individual reported having been exposed to in the past, the greater their chances of infection with cold viruses and, once infected, the greater their chances of developing a clinical cold. Both the risk of viral infection and the risk of developing clinical symptoms increased in direct proportion to the amount of stress.

      The correlation between stress, infection and illness was impressively strong. Individuals with the highest stress

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