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The common cold is not an equal-opportunity attacker.
Everyone knows it's contagious, yet some stay healthy or suffer only mild symptoms even when they're exposed to armies of sniffling kids and co-workers. Others seem to be knocked out by every passing bug.
Scientists are far from understanding everything about colds. But a growing pool of evidence suggests that personality, stress and social life all can influence healthy adults' vulnerability to cold symptoms.
Just like in kindergarten, those who ''play well with others'' are better off, says psychologist Sheldon Cohen of Carnegie-Mellon University in Pittsburgh. For 16 years he has been exposing volunteers to colds by dropping rhinoviruses into their noses. Then he quarantines them for five days to see who gets sick. Medical tests and questionnaires show:
* Happy, relaxed people are more resistant to illness than those who tend to be unhappy or tense. Adults with the worst scores for calmness and positive mood are about three times more likely to get colds than the most relaxed and contented adults. When happy people do get sick, their symptoms are milder.
* The more extroverted a person is, the less likely he is to catch cold.
* Serious work-related or personal stress for at least a month increases the chances of catching cold. The longer people had lived with bad stress, the more likely they were to catch cold in the lab.
* Playing diverse social roles -- spouse, parent, worker, friend, club member -- improves resistance to infection. Those with three or fewer roles were four times more likely to get colds than those with six or more.
The stress factor
Pam Jacobson, 39, knows about stress and demands: She has three children ages 3 to 9, a husband who works long hours and a ''chauffeur'' schedule that traps her in Los Angeles traffic about two hours a day.
''I feel like I'm always juggling,'' she says. ''They all have different activities. I'm always on the way somewhere. I can barely remember things, I have to write everything down.''
Still, Jacobson is, by nature, an ''up'' and calm person, says her mother, Gloria Welles. ''Chaos can be going on all around her, but she's very steady.''
Although her kids bring home colds, and she gets them sometimes, symptoms tend to be mild, ''and I don't stop my life,'' Jacobson says. She attends yoga class twice a week to relieve stress and plays tennis. She also keeps up with pals. ''I have a lot of friends, and I make time to see them.''
The tie between human behavior and vulnerability to illness is not magical. Scientists are finding some explanations. For example, in a three-month study of daily moods and illness, adults had more antibodies to infection on days with positive events. The worse the day, the fewer antibodies, says psychologist Arthur Stone of State University of New York-Stony Brook.
Cohen believes a diverse social network can buffer against losses in any one area. And stress clearly affects the immune system, he says. Stress triggers the release of inflammatory particles, called cytokines and histamines, that fight off infection but also cause miserable cold symptoms.
About one-fourth of adults who are infected with a cold virus don't develop symptoms, and they have low levels of these inflammatory molecules, says Jack Gwaltney, an infectious-disease expert at the University of Virginia School of Medicine. Cohen is studying whether positive emotions affect release of these cytokines, perhaps easing symptoms.
In Stone's studies, adults got sick three to five days after their roughest days. Animal research may explain why people often get through acute stress but fall ill afterward, says Esther Sternberg, who directs the Integrative Neural-Immune Program at the National Institute of Mental Health.
Stress unleashes adrenaline-like hormones first, which stimulates the immune system. Then cortisol is released; this hormone battles inflammation but weakens immunity.
''For a while both hormones are going full-steam ahead,'' Sternberg says. When the stress ends, the adrenaline hormones shut down first, leaving hormones that suppress immunity to hang around alone, ''and in these few days illness may take hold.''
Conflict as a culprit
Miserable marriages can weaken the immune system and promote colds, adds psychologist Janice Kiecolt-Glaser of Ohio State University. In her study, newlyweds who used putdowns and sarcasm in their exchanges had more respiratory infections and poorer immune function in the first three years of marriage; wives were hit particularly hard.
''It's not always easy to get out of stressful situations,'' Cohen says. ''But it's clear that jobs and marriages with constant conflict are not good for your health. Not everyone is a positive person, but we all should try to be this way more.''
In the face of stress, ''try to pace yourself,'' Sternberg says. ''If you've been through an unusual challenge, take it easy for a while.''
That's especially important as people age because the immune system isn't as strong. ''We need to respect the limits of our bodies.''
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