It can happen in the back seat of an automobile or in the first class cabin of a trans-Atlantic flight. Wherever it strikes, an estimated 90 percent of American adults at some point in their lives meet that bane of travel: motion sickness.
For the 10 million to 15 million adults who encounter it regularly when they travel, dream vacations turn into nightmares. And the syndrome's debilitating symptoms are a constant threat to pilots, Navy crews and astronauts.
But now help is on the horizon.
Using contraptions that even an amusement-park fanatic would have trouble enjoying, scientists have recently identified many of the perceptual, neurological and hormonal components of this loathsome gastric disturbance.
The research could pay off with new drugs not only for motion sickness but also for morning sickness in pregnant women and for nausea in cancer patients on chemotherapy.
While many scientists now agree on an evolutionary explanation for why motion often leads to nausea, recent studies indicate that some people harbor genes that make them particularly susceptible to motion-induced queasiness.
And in research that has particular appeal to drug-shy tourists, scientists will report this summer that a technique resembling acupuncture without needles is a very effective means of preventing motion-induced nausea.
Most of the latest research is still too new to have an impact on this year's summer vacation. And while several drugs are available to prevent motion sickness, many a veteran traveler will testify that there is plenty of room for improvement.
The most effective drugs have a tendency to make people drowsy and can cause blurred vision, dizziness, nausea and vomiting -- the very symptoms they are meant to prevent.
"There are embarrassingly few good drugs for motion sickness," said Dr. Kenneth L. Koch, a gastroenterologist who specializes in the syndrome at Hershey Medical Center in Pennsylvania.
Although associated today with modern modes of travel, motion sickness is an age-old problem. The word nausea has roots in the ancient Greek word for ship, and Hippocrates linked motion sickness to jostling of the guts.
Napoleon is said to have abandoned plans for a dromedary corps in Egypt after being advised of the ill effects of the camel's gait on the rider's stomach.
But motion sickness turns out to be more in the mind than in the stomach. The problem, said Dr. Koch, is one of a sensory mismatch in the brain in which the vestibular system of the inner ear sends messages about body position and movement that contradict information relayed by the eyes.
Inside the cabin of a rocking boat, for example, the inner ear detects changes in linear and angular acceleration as the body bobs with the movement of the boat. But since the cabin moves with the passenger, the eyes register a relatively stable scene.
Agitated by this perceptual incongruity, the brain responds with a cascade of stress-related hormones that can ultimately lead to nausea and vomiting.
Similarly, astronauts in orbit can see that they are drifting about in the cabin of their spacecraft. But their inner ears, which cannot detect motion without gravity, remain convinced that the body is perfectly still.
To study the physiology of motion sickness under controlled conditions, Dr. Koch has worked with a Penn State psychologist, Dr. Robert M. Stern, and others, using a device that induces an illusion of motion.
Subjects sit on a stool inside a large vertical drum that has black and white vertical stripes painted on the inside.
The drum rotates around the individual about once every six seconds, and in less than a minute the person develops a visual sensation that it is he or she, rather than the drum, that is moving.
At the same time, the inner ear and various pressure receptors in the body tell the brain that the body is sitting still.
By asking subjects to report their feelings during a period of up to 15 minutes, and by measuring neurooendocrine activity and gastric muscle stimulation with catheters and fixed electrodes, scientists have created a detailed picture of the events that lead up to the headache, the sweaty palms, the disequilibrium and the rising thickness at the back of the throat.
They have found that even before feelings of nausea arise, the physiological responses of those who will get sick differ from those of people who will not.
Those who will develop motion-induced nausea secrete increasing amounts of the stress hormones epinephrine and norepinephrine early in the onset of a motion experiment.
After a few more minutes, levels of another hormone, vasopressin, climb to very high levels in the blood, and electrical rhythms in the stomach muscles "go bonkers," Dr. Koch said.
Instead of the usual three waves of electricity that typically lumber from the top of the stomach down to the small intestine each minute, the rate accelerates to as high as nine per minute.
Ultimately, said Dr. Koch, "the body says, 'OK, we can't take this anymore. We have to do something.' " The stomach engages in that all too familiar series of vigorous convulsions known in scientific vernacular as "reverse peristalsis."