TRAVEL RULES THAT I HAVE LEARNED, sometimes painfully: Eat no unwashed fruits and vegetables, and no washed ones, either. Salmonella bacteria can persist deep in the crinkles of a lettuce leaf. No fruits or vegetables should be consumed unless they're cooked to a fare-thee-well. Ditto all other food, basically. If it's really cooked, it's probably not bad. Bread is usually OK; also cookies. In general I try not to eat seafood except in the U.S. (and I've gotten horribly sick on it once here, come to think of it). Never drink raw milk, though it looks tasty and bucolic. Carry Wash'n Dris and wash your hands frequently. Once, after visiting the men's room (to return to that grim subject) in the Omsk airport, I used about eight Wash'n Dris, then for good measure asked my Russian friend to pour straight alcohol all over my hands as well as on the soles of my shoes. My God, that place. I never saw anything so gross.

Alcohol, in strengths you drink, does not purify anything. Ice in drinks is always a bad idea; luckily, foreigners aren't as in love with iced beverages as we are. Boiled water is safe for drinking, but opinion differs on the amount of boiling required. I've heard that water needs merely to be brought to a boil, and also that it should boil vigorously for ten minutes at the very least. A travel doctor I consulted before a trip to the Russian Far East strongly recommended the ten-minute rule, and I promised myself I'd abide by it—not a practical plan, as it turned out. After an open-boat journey in wind and rain in early fall with two indigenous Chukchi guys across a fjordlike body of water on the Chukchi Peninsula, the guys beached on a rocky spit, tipped the boat on its side, got under it, took out a blowtorch, and lit it. Then they applied the blowtorch to a teapot, quickly boiled water, made tea, and offered me a cup. At that point I could not really request that they blowtorch it for ten minutes more.

The travel doctor I saw also told me to drink only bottled beverages if possible. "You can get bottled water everywhere," he assured me. Conversations in a clean, quiet anglophone doctor's office in North America don't always give you a clear idea of the actual situations awaiting you. You cannot get bottled water everywhere. In fact, the purpose of many journeys is to go beyond the places where you can. In Russia, knockoffs exist of many products. There's even a watery version of Johnson's Baby Shampoo, eerily similar to the original, down to the bottle and scent. Of course, there's fake bottled water in Russia, too. Russians sometimes laugh at people who insist on bottled water only, and tell them it's just tap water with a cap on.

For more advice on such subjects, I recently called Dr. Mark Wise, graduate of Britain's London School of Hygiene and Tropical Medicine, director of a travel clinic in Toronto, and author of a book about staying healthy while you travel. On the water question, Wise recommends buying bottled water in places where you can see many bottles of it and cases of it still sealed. Failing that, buy fizzy water. "Carbonation is harder to forge," he says. As to boiling, Wise is a bring-to-a-boiler; two minutes of a rolling boil kills most bacteria and parasites, while boiling longer wastes water and fuel at a tiny increase of safety. The Canadian-made water-purifying tablet with the brand name Pristine tastes better than iodine, he says, and may be used when no other means of purification are available. "Sometimes, though, you just have to trust what common sense says is OK," he adds. "You can also go too far being paranoid."

Wise became a travel doctor because of the movie Butch Cassidy and the Sundance Kid, which inspired him to take a backpacking trip around South America. Contemplating a volcano in Ecuador while simultaneously enduring an acute onset of the common traveler's ailment concentrated his mind on what could be done for the suffering traveler. Today, Wise is the travel-medicine consultant for several nongovernmental organizations as well as for a number of unaffiliated travelers, and at any one time he has many patients scattered in different parts of the world who are in touch with him by e-mail. "Even on short journeys, a third to a half of all travelers to the poorer parts of the world will get sick," he says. "Usually it's with stomach or bowel problems from dirty water or dirty food. If you can keep hydrated, stay near a bathroom, and let the illness run its course, usually it clears up by itself in a few days. But people e-mail me about many other health problems, too—malaria, animal bites, vehicle accidents, falls. I had a fall myself a few months ago while traveling in Ghana—stepped into a dry, dark sewer as I was walking along at night and screwed up my leg and ankle."

A traveler's problem that Wise deals with more often than you might expect is psychosis. "Psychotic episodes are more common in the younger age group," he says. "The antimalarial drug mefloquine can contribute to them. A kid will be in a faraway country, in a strange environment, maybe taking medications which can cause hallucinations, and he or she will lose touch with reality. These episodes sometimes occur even without an obvious chemical cause. Sometimes we have to send a psychiatric nurse to take care of the person and get him or her out of there. That's one reason I tell people they can't travel without medevac insurance. In a worst-case scenario, you want to be able to have your mother fly over and bring you home.

"When you're sick in a foreign country, there's generally a lot of chatter around you," Wise continues. "You may be treated with presumptive medicine, where a doctor or somebody guesses at the problem and tries to cure you by polypharmacy. Or else people tell you misleading things—local things, herbal things to try. None of that is necessarily bad, but it clutters and confuses your mind. You want to be sure it doesn't cause you to miss something important. In the midst of all that, getting in touch with North American medicine, even by a simple e-mail, can be a bit of a lifeline."

I know what Wise means. Global culture, which carries you along when you travel, is powerful but blunt. It doesn't bother about details, especially human details. As we pass from country to country, we probably have no idea what the people around us are saying, and what they're thinking is ten times more mysterious. Often the best we can do is listen for tones—anger, joy, fear—the way animals do. This ad hoc approach is unreliable and frustrating. I've spent weeks among people where my grasp of what was going on with them, and vice versa, was a scrambled TV signal almost constantly. The mutual obtuseness wears everybody down and makes them mad. When you get sick, your condition isn't generic but specific. Not getting it exactly right can have frightening consequences.