Doctors need to know exactly what patients did in a foreign country: Did they stay in a city penthouse, mingle with the locals in villages, lie on grass where ticks might lurk, swim in a parasite-laden river or lake?

Visitors to exotic locales have long been warned not to drink the water. But tourists also face plenty of other health dangers -- including food, mosquitoes and bugs on the ground -- and much of the travel advice is based on data from the 1980s or small, one-country studies.

Now, the records of ill travelers treated at a network of 30 travel-medicine clinics on six continents, called GeoSentinel, have yielded the most comprehensive picture yet of the illnesses most likely to strike visitors to particular regions of the Third World.

"This is a real blueprint" for doctors, said Dr. David Freedman, lead researcher of a study reported in Thursday's New England Journal of Medicine. "Where the traveler has returned from really determines ... what diagnoses you should worry about and what you should test for."

Problems May Not Show Up Right Away

Each year, about 8 percent of the more than 50 million travelers to developing countries become sick enough to seek healthcare during their trip or when they return home. Depending on the destination, up to two-thirds become sick, most with short-lived diarrhea, skin problems and respiratory infections.

And foreign travel, including business trips and immigrants' visits back home, is on the rise, with more than 760 million people crossing borders in 2004.

GeoSentinel's records on 17,353 ill tourists treated from 1996 through 2004, after their return home from 230 developing nations, show many illnesses were not apparent for a while: More than one-third of the patients became sick over a month after they got back, and one in 10 fell ill more than six months later.

The records showed many had lingering diarrhea from infections by parasites, now more common than bacterial diarrhea; dengue fever has become more prevalent than malaria in most regions; and infections from tick bites are now a big problem in sub-Saharan Africa. GeoSentinel shares its findings with health agencies so that they can update travel recommendations.

American Doctors in the Dark

American doctors rarely see and often fail to recognize some of these exotic diseases, or diagnose them too late.

Freedman, director of the Traveler's Health Clinic at University of Alabama at Birmingham, said doctors treating a patient with puzzling symptoms should ask about foreign travel, and patients should volunteer the information.

Dr. Rajendra Kapila, an infectious disease specialist at University of Medicine and Dentistry of New Jersey in Newark, said the database provides good clues for doctors to follow. But he said doctors still need to know exactly what patients did in a foreign country: Did they stay in a city penthouse, mingle with the locals in villages, lie on grass where ticks might lurk, swim in a parasite-laden river or lake?

Outside poor countries, travelers also face health hazards. Hotels in warm Mediterranean countries are linked to Legionnaires' disease and cruise ships to outbreaks of viruses that cause diarrhea and vomiting, Dr. David R. Hill, director of Britain's National Travel Health Network and Center, wrote in an editorial.

Dr. Bradley Connor, president of the International Society of Travel Medicine, said data from the study can help remind travelers to take such precautions before departure as getting vaccinations or packing anti-malaria pills.

"People spend a lot of time researching hotels and airfares, and they don't give much thought to the health issues that could potentially ruin the trip," he said.