WHAT UNDOES YOU, USUALLY, is what you gulp. Once, in a bar in Montana, I held my hand against the glass of a case containing a coiled and rattling rattlesnake; if I could keep my hand against the glass when the snake struck, the bartender said, he would buy me a beer. The bet proved a safe one on his part, because it is in fact impossible not to flinch from a striking snake, no matter how thick the intervening glass or strong your will. When you travel, a similar deeply conditioned response occurs: You know the food is bad, the water dangerous. You discipline yourself consciously, stick to bottled water, eat only the mildest, most scrutinized food. Vigilance carries you through one trip, maybe several. But then in an unguarded moment you're hungry, you're thirsty, something smells good, the water from the spring looks refreshing, your reflexes take over—and you gulp.

(Although, strangely, what sickened me that time in Russia was not gulping but caution. I had been so careful about what I drank, I became dehydrated from not drinking enough.)

And the result of gulping, most of the time, is that you come down with a certain ailment. This ailment is in fact the most common one that travelers get. It is so well

You discipline yourself, stick to bottled water. Vigilance carries you through one trip, maybe several. But then in an unguarded moment you're hungry, something smells good, the water from the spring looks refreshing, your reflexes take over—and you gulp.

known that it does not need to be named; you know the ailment I mean. Usually it is caused by food. It is disagreeable and inconvenient but not serious. It sneaks up on you no matter how careful you are. Sufferers usually recover from the worst of it in two to three days. In its commonest form I don't even consider it an illness, technically. It will not kill you. What does kill travelers is accidents (if you're young) and preexisting medical conditions (if you're old). These problems are rarer, thankfully. But in almost all travel stories, an unmentioned amount of the most common travel ailment can usually be assumed.

People who described journeys in former times, when one refrained from being gross in polite company or in writing, indicated attacks of the ailment with the word indisposed. A caravan trekker who crossed Asia, reached the Great Wall, entered China, and then ate his first-ever Chinese banquet might report that after the feast he "suffered a most severe indisposition." Many notable journeys involved these indispositions, usually left more or less blank in the tale.

Francis Parkman, a Boston Brahmin and Harvard grad, went west the summer after finishing law school, in 1846, to observe and live with Indians on the Plains. When he and his companion, Quincy Adams Shaw, reached Fort Laramie, in present Wyoming, Parkman became ill. His "disorder" was so severe he could hardly sit his horse, and of course he made slow progress toward the Sioux camp, having to dismount all the time. Parkman kept riding, on to the Black Hills, so ill at moments out there in the wilderness that he feared he would die. His intrepidity and persistence not only outlasted the ailment but made his book—and, arguably, his later career as a great American historian. Shaw, for his part, came down with poison ivy and spent much of the journey back at the fort "lying on a buffalo robe . . . solacing his woes with tobacco and Shakespeare."

At least Parkman didn't have to deal with the great discrepancy in bathrooms that exists today. Presumably, using the outhouse at Fort Laramie (not to mention simply the bushes) did not differ a lot from similar necessities anywhere. Nowadays, though, if you leave America and travel to someplace interesting like Asia or Africa or South America (I'm sorry, but I don't consider most parts of Europe interesting), you encounter bathrooms that are an unimaginable nightmare. Most countries that are not America occupy a different universe, bathroomwise. A friend who travels in China and Nepal says that what the U.S. should send to foreign lands is not Peace Corps volunteers or World Bank economists but plumbers and plumbing supplies. In northern parts of Russia, where winter temperatures go to 45 below, liquids freeze so quickly that a kind of stalagmite effect occurs in the outhouse, rising up through the hole in the floor. . . . As the poets say, let us draw a veil across the scene.

The common traveler's ailment causes many of us to see more of this sort of thing than we're really interested in. But, as with a lot of unfortunate circumstances in real life, it could get worse. You could eat bad fish in Thailand and get liver flukes. I'm not even sure what liver flukes are, but you don't want them. You could consume a generous helping of a coral-reef fish that has been feeding on crustaceans rich in some bizarre nerve toxin, and absorb the toxin, and fall into a zombie state (real zombies ingest this toxin deliberately, I believe, in order to become zombies), and lie in a near-death paralysis for who knows how long, as happened to the novelist Saul Bellow. In his seventies at the time, the Nobel Prize winner went out to dinner on the island of St. Maarten, came back to the hotel, felt poorly, and lay catatonic for months. Don't order the grouper is the moral here.

So many maladies for you to catch! Malaria is still quite active in equatorial climates, and drug-resistant strains exist now. Web sites devoted to the disease feature constantly updated maps showing the worst malarial zones. Cholera, a frightening-sounding illness, waits in drinking water and kills by dehydration; if you get it, and can keep hydrated and take antibiotics, it goes away in about a week. Tick-borne illnesses thrive all over, in temperate as well as tropical lands. Serious versions of Lyme disease that can put you in the hospital may be found no farther away than upstate New York. In Russia, a certain kind of tick that lives east of the Ural Mountains carries a fever for which there is no cure. Camping in the summer in Siberia, I awoke one morning to find a tick—the Russian word is pronounced "kleshch," a triumph of onomatopoeia—kleshching to me. I showed it to my Russian companions and they assured me that though it was indeed the kind of tick that carries the fever, the insect is contagious with it only in the spring.

Giardia, a troublemaking intestinal parasite, does well in much of India and can be in untreated water almost everywhere. It turns up sometimes in the tap water in St. Petersburg. A tricky aspect of giardiasis is the time-lapse way it presents: You think at first you merely have the common traveler's ailment, until it goes on too long. Schistosomiasis, another parasitic disease, also waterborne, has similar symptoms. A swim in the Yangtze or the Mekong may give you a dose of it. Dysentery also resembles the common traveler's illness, only it involves blood. Typhoid, hepatitis, polio, SARS, avian flu—I'm getting depressed—and of course the various STDs threaten travelers. A male flight attendant for Air Canada was supposedly the first major international transmitter of AIDS. Even watching the Travel Channel and eating snacks causes arteries to harden and plaque to accumulate in your veins. There is no real hope anywhere.