As the chill of winter arrives, so does the prospect of congestion and coughing, sore throats and sniffles, and other less-than-desirable symptoms.

But you might have noticed that colds, flus, and other seasonal blights seem to hit some people again and again, while others coast through to spring unscathed. Is this punishment random, like a tornado, or is there a method to the cold-and-flu madness?

There is indeed a method. While we can all help prevent viral and bacterial infections by washing hands regularly and avoiding germ-laden surfaces, it’s not just the bugs we encounter that determine how healthy we are. Our well-being also depends on how prepared our bodies are to fight them.

An entire field of research called psychoneuroimmunology has bloomed in recent years. It aims to uncover the links between the brain, behavior, and the immune system. One of its primary targets is the universal phenomenon known as stress.

We’ve all experienced the nasty cold that comes after a deadline, and scientific evidence has borne out this pattern, with clinical studies showing that psychological stress can weaken defenses and slow recovery from illnesses small and large. A recent study from the University of California, Los Angeles, in fact, found that the effects of stress play out at the cellular level: Hormones cued by emotional stress make immune cells age faster.

What is stress? “It’s our body’s response to something that taxes or exceeds our resources, whether from external pressures or our own internal worries,” says Frances Cohen, a psychologist at the University of California, San Francisco. What feels stressful depends on a person’s reaction to a given circumstance. “For some people, getting divorced is a positive step,” she says. “But for others, it’s devastating.” When we perceive something as harmful, our brain triggers hormones such as cortisol, which can affect the immune system. “You can trace it from the brain through the body,” Cohen says.

After stress hormones pour into the bloodstream, “the body tries to correct itself, to bring itself back into homeostasis,” she adds. So if the stress is brief, the hormones dissipate, and the body returns to normal. In one study, her group found that people who lost their jobs had lower levels of an important class of immune cell. However, she says, “we saw a recovery of immune levels, a bouncing back, in people who got a new job.”

Stress that lasts over a long period of time proves far more damaging to our bodies than short-term stress. A study published earlier this year, for instance, found that consistently high levels of perceived stress could reduce women’s ability to fight an infection from human papilloma virus (HPV), which can cause cervical cancer; no such effect occurred with one-time stressful events such as job loss.

And in a preliminary study published in August, researchers studied people caring for family members with chronic illnesses (a long-term stressor); compared with a control group, the caregivers’ white blood cells were less responsive to cortisol and more responsive to a pro-inflammatory compound — leaving their bodies in a state of chronic inflammation.

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