ATLANTA - I'm not getting sick. I'm not getting sick. I'm about to go on vacation, and I'm not getting sick.

That's what you're telling yourself because you bought the tickets months ago, you've worked a lot of overtime, and you deserve some R&R. So you've decided to ignore that slight sniffle, hovering headache, and raw throat. No matter how congested you are, you're going to board that plane and try not to sneeze on the lady next to you in the aisle seat.

The last thing anybody wants to do is spend his or her scheduled time off being ill - especially these days, when many employers lump vacation and sick time into one pot, and unexpected stomach flu can swallow time saved for a getaway.

And nobody wants to be the person who calls in sick the day he's supposed to return to work from a week at the cabin. Even when you really are under the weather, it just looks bad.

More than a few of us have legitimately been that person. So why does the body seem to break down just when it's about to take a well-deserved rest?

Researchers who study the impact of stress on the immune system say it's not all in our heads. Vacations don't make us sick, but the stress we incur leading up to our time off sets us up for illness right about the time we're unfolding the beach blankets.

"Stress before you go disregulates your immune system, so it's priming you for bad things," says clinical health psychologist Janice Kiecolt-Glaser, the S. Robert Davis chair of medicine at Ohio State University's College of Medicine.

Kiecolt-Glaser and her husband, Ronald Glaser, are recognized as pioneers in stress/immunology research, linking stress to a diminishing of the body's capacity to heal wounds and shortened life expectancy for the caregivers of terminally ill people.

In the late 1980s and early 1990s, the Glasers did a clinical study with medical students to determine the effects of stress on the immune system. They took blood twice a day from the students to track their immune systems for an entire school year.

The fact that tests for a semester were usually announced in advance gave the Glasers benchmarks by which to measure how the students' bodies responded to serious deadlines.

The Glasers found that before taking a scheduled test, the students would sleep less, eat poorly, and wear themselves down with all-night study sessions. In turn, their blood work showed their immune systems were not functioning at a normal level.

"Why it's relevant for the rest of us is that several days before you're about to take a vacation, those days are usually filled with frenzied activity where you're trying to pack in a week or two's worth of work into a couple of days," Kiecolt-Glaser said. "That's the same type of overload our students experienced."

Cynthia Douglas was looking forward to a week without being tied to her BlackBerry. A corporate communications manager for an airline, Douglas, who lives in Alpharetta, Ga., worked right up until her family left for spring break in Seaside, Fla., even though she felt a cold creeping up.

Her symptoms seemed to subside while she soaked up the sun, "but the minute I got back, I got sick with a sinus infection. And I've had that happen several times before," Douglas says. "It makes you wonder: Do you just will it, or is there a medical reason for it?"

That ability to power through an illness - at least initially - has roots in the same fight-or-flight response the body has when it is threatened with danger, says Marc Schoen, an assistant clinical professor of medicine at the UCLA School of Medicine.

"When the body is under stress, there is an adrenal response that helps mobilize the body," Schoen said. "It's that same impulse that helps us meet a deadline."

Which could help to explain why despite the scratchy throat, we keep pushing ourselves because we can see a few days off just over the horizon.

"The mind can get itself through things," says Charles Raison, clinical director of the Mind Body Program at the Emory University School of Medicine in Atlanta. "Like people who want to live to 100, and they do it, only to die two weeks later."

Earlier this year, Ken Thorne of McDonough, Ga., had a death in his family just 10 days before he was scheduled to go on a Boy Scout leadership-training event near Cape Canaveral, Fla.

An analyst for a cellphone company, Thorne made it through the funeral, but within days of his return, he felt as though he was coming down with something. He went back to work, then left a couple of days later for the leadership training in Florida.

"Basically, I spent my time down there in Florida being sick with a viral infection," Thorne says. "At the time, I didn't realize that having the death in the family would be so stressful."

As much as a vacation would seem to lower a person's tension, it also can raise the risk of picking up a bug.

Generally, people have more resistance to germs in their normal orbit of home, work, and play, Raison says. Once you reach your destination, there is a whole new set of germs waiting for you. And when you're stressed when you arrive, those germs probably will find you a ready host.

Jason Parker took his wife and daughter to visit family in Mississippi for Christmas. During the last couple of days of the trip, he got the sniffles. He thought it was the start of a regular cold. Once he got home to Atlanta, he felt green at the gills.

"I lost the last four days of my vacation," Parker says. "We'd even gotten a babysitter for our daughter so that we could go to a party on New Year's Eve. But that night, I was lying on the couch with over-the-counter medicine and Kleenex, watching the ball drop while my wife went out to the party."

A few days later, Parker tried to go back to work, not wanting to call in sick on the day he was supposed to be back. He made it halfway through the day when his boss told him to go home before he contaminated everybody else in the office. It turned out he had influenza.

So how do you avoid such potential time-off spoilers? With many companies downsizing the number of employees but not the amount of work, how can you avoid the kind of stress that will make you sick?

The answer is as straightforward as a bowl of chicken soup.

"Poor sleep disregulates many different aspects of the immune system," Kiecolt-Glaser says. "The time you're preparing for a vacation is probably the time you're going to exercise the least. And when you're stressed, you want to eat more Twinkies, not vegetables.

"Everything your mother told you still makes sense - sleep, exercise, a balanced diet."