By JANE E. BRODY

Explanations of why people catch colds are almost as numerous as the viruses that cause colds. They range from the environmental -- living with small children, riding the subway at rush hour, getting chilled to the bone -- to the personal -- smoking too much, exercising too little, sleeping poorly, eating erratically, working too hard.

But studies under way at Carnegie Mellon University in Pittsburgh suggest that psychological stress is also a very important factor in determining who gets sick when nasal passages are invaded by a cold-causing virus. Just any old stress will not do. It has to be long-term stress, lasting at least a month and stemming from a significant problem like being fired from a job after years of service or being left financially or emotionally bereft by a divorce. The researchers point out that stress is not the cause of all colds. Rather, people under severe stress are more likely to catch cold when exposed to a virus than people under milder stress.

Dr. Sheldon Cohen, a psychologist at the university, has spent years trying to discover why some people frequently catch colds, while others rarely get a sniffle. In 1991, he directed a study of 394 men and women that identified psychological stress as an important factor in colds.

He and co-workers in Britain showed that the higher a person's stress score on a standard test, the more likely the person was to develop a cold when exposed to a cold virus. Stress was an important risk factor even when smoking, lack of exercise, poor diet, disturbed sleep and alcohol consumption were taken into account.

In the current studies, financed by the National Institute of Mental Health, Dr. Cohen and colleagues at the University of Pittsburgh School of Medicine and the University of Virginia Health Sciences Center subjected 276 healthy volunteers ages 18 to 55 to physical, social and psychological examinations before placing them in quarantine and depositing cold viruses in their nasal passages. On each of the next five days, volunteers, paid $800 each, were examined to determine who became infected by the virus and who then developed symptoms of a cold.

Last June in the Journal of the American Medical Association, the team reported that the volunteers with the most ties to relatives, friends and community were the least likely to catch a cold. The relationship between having many social connections and being relatively immune to colds held even though cold viruses spread easily among people.

Although this finding would seem counterintuitive, Dr. Cohen said that other researchers also have found that ''having many different kinds of social relationships helps to protect against disease.'' The message from this study: ''Be involved and participate in your community'' to increase your chances of staying healthy, Dr. Cohen said.

The newest findings, published in the May issue of Health Psychology, a journal of the American Psychological Association, confirmed the earlier study showing a strong link between susceptibility to colds and stress. But this time Dr. Cohen sought to determine the kinds of stress involved and how they might affect resistance to colds. The study showed that only chronic stress, lasting a month or more, affected the risk of catching a cold and that two causes of stress -- being unemployed or underemployed, or having interpersonal difficulties with relatives or friends -- had the greatest influence on risk.

Being under severe stress for more than one month but less than six months doubled a person's risk of a cold, compared with people experiencing only routine stress. Stress lasting more than two years nearly quadrupled the risk. Likewise, the stress of interpersonal difficulties doubled the risk of a cold, and being under work-related stress raised the risk 3 1/2 times. However, less common stresses had no effect on participants' chances of developing a cold.

Even being socially well-connected, which could provide emotional support during hard times, could not overcome the harmful effects of chronic severe stress, the researchers reported.

But when Dr. Cohen and colleagues looked for a biological explanation, they were surprised to find that increases in the stress hormones epinephrine and norepinephrine could not account for the strong relationship between stress and colds. Similarly, blood levels of natural killer cells, which constantly search the body for abnormal cells and wipe them out, were affected very little.

So now, Dr. Cohen and researchers are looking at substances called cytokines that have an indirect effect on tissues that are being invaded. Cytokines are messenger chemicals of the immune system that travel through the blood and send out an inflammatory alarm when cellular abnormalities are discovered. The alarm marshals macrophages and other reinforcements to battle the invader. This response of the body to a viral infection, not the virus itself, causes the sneezes, congestion, runny nose and other cold symptoms.

Source：The New York Times