Chronic stress, whether it stems from external pressure or internal perception, also impairs immunity. People who experience higher levels of stress or negative moods are more susceptible to infection, develop more-severe illness, and take longer to heal than their mellower counterparts, research suggests.

For example, researchers at Ohio State University created blisters on the arms of 98 participants and monitored how quickly the wounds healed. The subjects also completed questionnaires assessing how they expressed their anger.

Two-thirds of those who managed their anger well healed within four days, compared with one-third who didn't control their anger. Other research suggests that steps that ease stress, including the techniques described below, may enhance immunity.

Tai chi. Older people who learned this martial art, which combines exercise, relaxation, and meditation, showed increased immunity to the shingles virus, according to a 2007 study in the Journal of the American Geriatrics Society.

Relaxation training. Researchers from Ohio State University randomly assigned 112 women undergoing chemotherapy after breast-cancer surgery to an intervention program that included progressive muscle relaxation (in which you tighten and then relax your muscles) and 113 women to a control group that received no special treatment. The intervention group experienced reduced stress and an increase in the ability of antibody-forming T cells to multiply, while the control group had declines in those cells.

Meditation, as well as a combination of guided imagery and self-hypnosis, may also improve immunity.

Social support. Ovarian-cancer patients who had greater social support had higher levels of natural killer-cell activity than patients with lower levels of support, according to a 2005 study in the Journal of Clinical Oncology.

Other studies have shown that people who regularly attend religious services have slightly stronger immune systems, and those who have a thriving network of relatives, friends, and acquaintances might have a lower risk of catching colds.

Positive attitude. People who maintain optimism or a sense of humor during stressful situations may have moreactive immune systems or higher levels of disease-fighting antibodies than gloomier people, several studies suggest.

Massage. It may boost immune function in women treated for breast cancer, which could help prevent a recurrence. Researchers suspect that any activity that helps you relax or improves your mood, including yoga, reading a good book, or enjoying a hobby, probably has similar benefits.