Stress puts a strain on the body's immune system

(CNN) -- The body's immune system is a true fighter -- protecting people from disease by keeping viruses and bacteria from taking over and ruining our health.

But one thing the immune system can't always fight is stress. Studies have shown that a stress-ridden immune system operates on a weakened level, making the body more vulnerable to cold viruses and other diseases.

The effect can be even greater on patients whose immune systems are already compromised. Researchers have found that people infected with HIV, the human immunodeficiency virus, are at greater risk for the lethal disease AIDS. Another study showed severe stress increases a breast cancer survivor's risk of relapse -- by a factor of six times.

So why does how we think and feel and the kind of environment we're exposed to matter so much to our health? Psychologist Janice Kiecolt-Glaser said it has to do with the "balance" our immune system holds in fighting illness.

"Especially for people in their sixties and seventies (years of age), stress appears to matter a great deal more," said Kiecolt-Glaser, of Ohio State University. "But even for younger people, there's good evidence that even things like colds and flu can be enhanced by relatively minor stressful life events.

"When people are stressed, small alterations in immune function may actually help tip the scales and increase the odds" of someone getting sick, she said.

Stress can send the immune system into a tizzy, triggering the pituitary gland to release a hormone called ACTH. ACTH then stimulates the adrenal gland to release another hormone called cortisol, which attaches itself to the immune system's disease-fighting cells.

With a load of cortisol on its back, an immune cell has a harder time doing its job.

But not all stresses are created equal, experts said. Getting stuck in traffic, for instance, sounds and feels frustrating. But psychologists say travelers know the traffic crunch will be over -- and that helps to keep the immune system somewhat protected from stress.

Longer-term stresses like a divorce or a remembered traumatic event, however, can do a number on the body's ability to fight off infection. Keeping it alive in your mind, Kiecolt-Glaser said, allows stress to do battle with a person's immune system.

CNN Medical Correspondent Elizabeth Cohen contributed to this report