A trek through the Amazon jungle or an African safari may hold appeal for adventurous travelers weary of Europe. But the lure of exotic, faraway places holds hidden dangers for the unprepared tourist who travels without first receiving the proper immunizations.

According to Dr. Phillip F. Pierce, director of the International Health Service at Georgetown University Hospital, people traveling to the developing world should get medical advice six weeks before departure. The type of vaccinations needed depends on what country the person plans to visit, the time of year, duration of travel and type of accommodations.

Generally in visiting the Third World, travelers should update their childhood immunizations such as tetanus, polio, measles and rubella. "There was a recent case of a woman in Asia who came back with paralytic polio despite having childhood vaccinations," Pierce said. "It seems that it {the effectiveness} wanes after 20 years."

In addition, it is sometimes necessary to take precautions against the following illnesses: :: Malaria. Health authorities are particularly worried about malaria, an often fatal disease that can be prevented. It is most common in Africa, Central and South America and Southeast Asia.

During 1982 to 1984, 358 cases of malaria were reported to the U.S. Centers for Disease Control, Pierce said. Ten of those were fatal. "We have seen a tremendous upsurge at Georgetown, probably a case every other week," he said.

Malaria is transmitted by mosquitoes, which tend to bite at dawn and at dusk, said Dr. David M. Parenti, director of the Travelers Clinic at George Washington University Medical Center.

It takes 10 to 12 days after being bitten to develop symptoms. "A person on a two-week trip is not likely to get sick while traveling," said Dr. Martin S. Wolfe, director of Traveler's Medical Service of Washington. "They usually get sick when they come back." Symptoms include high fever, shaking, chills, headache, body sweats and pains. Malaria is easily mistaken for influenza or other viral and bacterial infections, Wolfe said.

The drug chloroquine -- commonly used to prevent and treat malaria -- kills three of the four malaria-causing parasites. The pills should be taken about a week before the trip and for four to six weeks after returning.

In Africa, Southeast Asia, certain South Pacific islands and the Amazon region of South America, one malaria parasite has developed a resistance to chloroquine. Fansidar, a sulfa compound, is used in those places. Some people, however, are sensitive to sulfa drugs, and seven have died of allergic reactions since 1982, said Wolfe.

Among common-sense precautions against malaria: Apply a mosquito repellant to exposed parts of the body; wear protective clothing; sleep in screened accommodations, and use a mosquito net. :: Diarrhea. Although not fatal, diarrhea is a common problem for travelers. Since this condition is caused by bacteria in the food and water, it is difficult for people to protect themselves, Pierce said.

General recommendations are to drink bottled water, avoid raw fruits and vegetables and stay away from street vendors. Carry Pepto-Bismol, either the liquid or tablet form, for mild cases. Doctors sometimes prescribe an antibiotic for people to take with them for severe cases. :: Hepatitis A. A shot of gamma globulin for hepatitis A -- good for four months -- is advised for people traveling to developing countries with poor hygiene. The disease, a viral infection of the liver, is contracted from food and water contaminated by feces. Although not generally fatal, it can be debilitating, causing some people to miss work for up to a month, Pierce said. The shot is generally not given to people staying in high-quality accommodations for a short time -- a week or so. :: Yellow fever. Shots are required for several countries in Africa and South America. Yellow fever is a viral infection transmitted by a mosquito and can be fatal. One shot lasts 10 years. :: Cholera and typhoid. Current vaccines for cholera and typhoid are not very effective. But these illnesses, though serious, are rare. "Since the risk for travelers is low," Parenti said, "we usually don't advise people to get them unless it is an international requirement." :: Meningitis. A vaccine against meningitis, a bacterial infection of the lining of the brain and the spinal fluid, is advised for travel to Nepal and parts of India. :: Encephalitis. People staying in Asia more than three weeks should receive a shot for Japanese-B encephalitis, a mosquito-borne virus that is almost always fatal if the brain is affected.

In general, experts say, travelers should: :: Wear an identifying bracelet to alert doctors to any conditions requiring emergency care. :: Carry a supply of any regular medication on the plane in case luggage is lost. :: Bring a first-aid kit. :: Drink bottled water, carbonated beverages, beer or wine; prepare coffee or tea from boiled water. Use ice only if the water supply is safe. Add products such as Globaline or Potable-Agua, which contain iodine, to purify water for drinking.More Information International Health Service at Georgetown University Hospital, 625-7379. Travelers Clinic at George Washington University Medical Center, 994-8466. Traveler's Medical Service of Washington, 2141 K St. NW, Suite 408, 466-8109.