"The entire violin section of the New York Philharmonic Orchestra disappeared in North Africa," said veteran tropical medicine specialist Dr. Benjamin H. Kean. "International conferences on tropical medicine were decimated in Istanbul and Teheran. Glee clubs lost their voices, and more, in South America . . . Above all, the United States lost the 1960 Olympics on the toilet seats of Rome."

Sooner or later, one out of every three people who cross a border will get travelers' diarrhea, the villian in each of the above episodes. It tends to hit younger people rather than older, usually starting about the third day of a trip or after. It is ubiquitous, principally in travelers going from industrialized to developing countries, and is known by a series of more or less inelegant soubriquets often connected with the geographic point of its origin: Turista, Delhi belly, Aztec two-step, G.I.s, Trots and, of course, the one rendered forever unforgettable by Jimmy Carter's gaffe: Montezuma's revenge.

It is an affliction of ancient vintage: Vasco da Gama and his explorers suffered. So, suggests the Old Testament, did the Philistines. "It did more damage to the crusaders than the Saracens," one speaker said.

But it was Dr. Robert Black, a University of Maryland Medical School epidemiologist, who, straight-faced, tracked the problem to its origins. "In what may have been the first occurrence of this phenomenon," he said, "it has been postulated that the dinosaurs became extinct because of the introduction of new diseases into previously stable and isolated populations. This may have been possible due to the receding of the oceans and the appearance of land bridges allowing the mixing of formerly separated species.

"It has been said that this first example of large-scale tourism during the cretaceous period resulted in the 'Great Cretaceous Diarrhea Attack' and ultimately the extinction of the dinosaurs.' "

Last week in Bethesda, the National Institutes of Health brought together some of the world's leading experts on this ancient and inelegant affliction to try to hammer out a consensus on its causes, avoidance, prevention and cure.

Quite a lot is known about it these days, although all the experts admitted that until recently it was marked as much by neglect as ubiquity. It is almost never fatal to the tourist, although diarrheal illnesses from many of the same microorganisms each year cause millions of deaths among children in developing countries.

However, as travel has burgeoned in the past few years, with 25 million U.S. citizens alone projected to travel to foreign parts this year, one third of them to developing countries, the search for the perfect preventative has intensified.

Published studies in the past few years have demonstrated the effectiveness of some preventive agents -- prescription or nonprescription -- and have created a thriving (and to some physicians alarming) commerce in certain antimicrobial drugs to be taken by the traveler every day. In addition, reports in medical journals that large quantitites of an 80-year-old patent medicine -- Pepto-Bismol -- could also fend off a travelers' diarrhea attack have given rise to medical concerns about its potential toxicity.

Despite evidence presented from an international body of researchers and clinicians, many demonstrating the effectiveness of these drugs in preventing travelers' diarrhea, the panel specifically recommended against their use, preferring to protect the two out of three travelers who would not get diarrhea anyway from the sometimes serious side effects of the drugs.

Neither the consensus panel nor the speakers, mostly clinicians and researchers, had a great deal to offer in the way of protection. Even the most prudent dietary precautions could not prevent all travelers' diarrhea attacks, although they did help. And even the best educated and most sophisticated world travelers admitted in a recent survey to succumbing to one or more dietary improprieties on their journeys. The survey was done among more than 600 Swiss tourists, 98 percent of whom admitted to one or more "dietary mistakes" leading to travelers' diarrhea.

However, the scientists agreed that once the diarrhea attack hits, some of the medicinal agents they rejected as preventives could shorten its duration and ease its symptoms. Moreover, because some antibiotics and other strong drugs regulated in this country are sold over the counter elsewhere, and because some of them have proven to be dangerous, the consensus statement recommended that travelers bring along their own. Among these:

Antimicrobials: Trimethoprim/sulfamethoxazole or trimethoprim alone or doxycycline, a tetracycline antibiotic. They kill the offending organism. Studies, including one still unpublished on doxycycline, suggest that TD attacks can be limited to less than 30 hours. Untreated, they usually last three to five days.

Antimotility agents: Synthetic opiate agents that slow the contractions of the intestines, thereby reducing symptoms. Two cited are diphenoxylate and loperamide. Because the organisms are not killed, there may be a relapse after initial relief of symptoms.

Bismuth subsalicylate: Principally Pepto-Bismol, which appears to work on mild cases in ways not clearly understood. New studies show the tablets work as well as the liquid, but there is a lag of a few hours before symptoms are relieved. Pepto-Bismol should not be taken with aspirin. Do's and Don'ts

Here are some specific precautions for traveling in countries where sanitary conditions are less than pristine. They may help prevent travelers' diarrhea, but there is no guarantee:

Do not drink tap water (although brushing teeth is probably okay).

Do not eat food bought from street vendors.

Do not eat raw fruit and vegetables already peeled. (Melon slices are big offenders -- unless you do the slicing.)

Do not eat salads.

Do not eat dairy products unless you know they have been pasteurized and kept cold.

Do use iodine tablets dropped into water and left standing for 30 minutes.

Do eat only at restaurants with reputations for cleanliness.

Do drink bottled drinks that are carbonated and flavored. (The carbonation increases the acidity, which lowers the possibility of contamination.)

Do wash hands frequently.

Veteran traveler and tropical medicine expert Kean adds this: "The only statement for which I have received universal acclaim is that on visits to some 65 countries I have never found a bad bottle of beer."