Being sick can be a bummer, but can being bummed out make you sick?

The answer is yes, according to new research that suggests a link between a lower mood and higher rates of infection.

Psychologist Sheldon Cohen of Carnegie Mellon University looked at how psychological factors such as social support networks, stress, emotional makeup and interpersonal conflict can affect immunity. In one study, his team evaluated the temperaments of 300 healthy test subjects and found that those who scored highest on the happiness scale were far less likely to develop colds.

When exposed to an infectious virus, in fact, those with the lowest happiness quotients were three times as likely to come down with a cold than their more upbeat peers.

Indeed, the confluence of holiday stress and seasonal affective disorders — lower moods linked to shorter days — contribute to increased cold and flu rates more than one might think, said Babette Wise, a licensed independent clinical social worker and an assistant professor of psychiatry at Medstar Georgetown University Hospital in Washington, D.C.

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“After the holidays, my phone rings off the hook with patients who are dealing not just with holiday stress, but with getting sick as well,” Wise told LifeZette.

One patient, a high-powered C-suite executive, “never had colds or other ailments while her husband was alive, even though the marriage was difficult.” Widowed now and alone after raising four children, she can’t keep colds at bay.

“She returned from a wonderful Thanksgiving break with her brothers and sisters. She started feeling sick with cold symptoms the minute she walked into her empty house,” said Wise. “The silence and the loneliness was overwhelming. After a few days of being back at her busy job, spending time with her dog and playing tennis with a friend, she started to feel better.”

The mood disorders that Wise treats increase a patient’s susceptibility not just to viral infections, but to other illnesses as well. One patient was dealing with a terrible marriage and myriad health problems.

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“She got her divorce papers and the same night ended up in the hospital with a kidney stone,” Wise recounted.  

At least 60 percent of her patients have some physical ailment, she said, which often improves or even disappears as they start feeling better emotionally.

People need to identify their unique stress triggers to help keep physical illness away.

Another patient became severely depressed after her son moved far from home. In a double whammy, the woman started to suffer debilitating back pains. Spine specialists couldn’t identify the problem, and no amount of medication or manipulation offered relief.

After a while, her gloom lifted, thanks to friends, therapy, and the tonic of time. As her mood improved, her back symptoms evaporated. It is a pattern Wise, in her many years of practice, has seen over and over again, she said.

People need to identify their unique stress triggers, whether that’s too much activity, loneliness, or excessive time on their electronic devices to help keep physical illness away,” said Wise.

Related: How Stressed Moms Fight Back

David C. Nieman, an immunity expert and professor at Appalachian State University, agreed but with caveats. He said that in the large-scale analyses he’s done, “the mental stress issue is there, but it’s not as important as exercise or eating fruit in terms of reducing cold risk.”

Sleep, interestingly, did not show up statistically as significantly impacting immunity. Emotional wellness did, yes, and diet, definitely, but for Nieman, the silver bullet is exercise.

“The most positive, powerful thing you can do to get well positioned for cold season is a brisk 30-minute walk most days of the week.”

This is especially important for young, highly educated females, the subpopulation Nieman found to have the highest cold and flu frequency.

Kristen Thames, a registered nurse with Cure Concierge Medicine in Malibu, California, fell into this category. Yet despite working in the medical field, she rarely got sick — until her marriage fell apart and the prospective challenges of single parenthood loomed.

Exercise, diet, sleep — she had all the bases covered. But during this emotional low point in her life, she said she “caught a chronic cold and cough that lingered for months.”

“I was going through a nasty divorce and was representing myself against the biggest law firm in Los Angeles, so I was stressed,” said Thames, the mother of three sons.

All of that is behind her now.

“I haven’t gotten sick in three years, despite never getting a flu shot, working in a doctor’s office and raising boys who bring home all kinds of germs,” she said.

Hand-washing, sleep, exercise, and diet are all important in warding off disease. But maintaining an “up” attitude also may help maintain a healthy immune function as well. In other words, add a little “ho, ho, ho” to avoid the “hack, hack, hack” of coughs and cold.