Now, a word on turista, variously known (depending on where you are or who you are) as Delhi Belly, Montezuma's Revenge, San Francisco-itis (by Mexicans traveling in America) or, simply, traveler's diarrhea.

Dr. James Curtin, chairman of the Department of Medicine at the Washington Hospital Center, says that the bacteria that causes it is more common than salmonella. It is a type of e-coli bacteria that can produce toxins that cause bowel cells to leak fluids. It is, he says, rather like attaching a mini-pump to the cells in the lower intestinal system.

The mechanism is not very different from cholera, but the traveler's variety is significantly less serious--rarely more than a nuisance for a day or two. It may involve running a low-grade fever along with the intestinal distress, nausea, headache and a general sense that this is no way to run a vacation; but it goes away by itself, usually in a day or so.

Doctors really do not know why some people are not bothered at all and others are laid low but, says Dr. Curtin, "people who travel a great deal and people who live in underdeveloped, tropical or semitropical countries seem to build up some immunity." Mexicans tend to believe it is one more crazy psychosomatic ailment of the crazy Norteamericanos.

It is widely recommended that Americans traveling in Mexico avoid ice and unbottled water even for brushing teeth. Personally, after three days of meticulous care (and relative discomfort caused by the lack of cold drinks on very hot days), I came down with a day-long siege anyway. Afterward I alternated iced tea with Pepto Bismol and had no major problems.

The fact is that the culprit is more likely a milk product than water or ice, says Dr. Curtin. Besides such obvious common-sense precautions as not buying anything edible or drinkable from sidewalk vendors and not drinking tap water, there are some more or less effective things to do.

One thing you should NOT do, says Dr. Curtin, is take antibiotics as a means of preventing the bug doctors call "toxigenic e-coli diarrhea." One reason is that the bacteria quickly develop strains resistant to all antibiotics; another is that you may be making yourself more susceptible to more serious ailments like salmonella.

There are two alternatives that have proven widely successful, although not for everyone:

Diphenoxylate-atropine, often sold under the trade name of Lomotil, is effective in halting the diarrhea. However, it is a strong codeine-related narcotic and overdoses have been known to virtually paralyze the intestinal tract. It is also likely to act as a knock-out pill, as narcotics tend to do, and there is some evidence that it may even prolong the bout by delaying excretion of bacteria and toxins.

Bismuth subsalicylate (sold as Pepto Bismol) is effective, especially in its liquid form, both as a preventative and as an aid during a siege. Drug stores (farmacias) in Mexico City have plenty of Pepto Bismol so you don't have to schlep bottles and bottles with you. Dr. Curtin suggests that an ameliorative dose during a bout would be one or two ounces every 30 minutes for eight doses. A couple of ounces a day does well as a preventative.

"It's too bad," he says, "that they make it so sweet."

Never mind. It works.