Health officials returning from the first international conference on AIDS in Latin America are privately pessimistic that the disease can be stopped from spreading relatively rapidly throughout the region.

Although the epidemic there is two to three years behind the United States -- which means there is time to set up public education campaigns to stem the spread of the virus -- many AIDS experts are concerned that cultural divisions, the lack of medical services and high levels of poverty present formidable obstacles to effective prevention programs.

If AIDS is not stopped, it could wipe out the gains of recent years made in child health and cause political and economic instability as the lethal disease strikes the most productive age groups in a population, those in their twenties, thirties and forties, who make the largest contributions in economic development, political leadership and creativity.

The Americas contain nearly 80 percent of the world's 50,000 reported AIDS cases -- the bulk of them in the United States. But Latin America is catching up, and about 12.5 million people in Latin America are believed to be infected with the AIDS virus, said Dr. Carlyle Guerra de Maceda, director of the Pan American Health Organization.

To stop the disease without having a working vaccine, public health officials must get people to change their sexual behavior, the most intimate and personal of all human activities.

"Precious little is known about sexual behavior in Latin America," said Dr. Ronald St. John, director of the Pan American Health Organization's AIDS program. "I wonder sometimes at us trying to change something that we know nothing about."

Changing sexual behavior has not been easy in the United States. Attempts by former prostitutes and by social workers to teach Washington, D.C., prostitutes about safe sex have had mixed results. Instead of stimulating condom use, the education campaign led many prostitutes to raise the price of sex without a condom by about $10.

Even in the U.S. homosexual community, where educational campaigns have been the most extensive, failures remain significant. "We still see homosexual men becoming infected with the AIDS virus," said Dr. Thomas Quinn, an AIDS epidemiologist with the National Institute of Allergy and Infectious Disease and the Johns Hopkins Medical Institutions. "They still are not listening. They are not using safe sex 100 percent of the time.

"People are going to have unprotected sex from time to time despite the best control programs," Quinn said. "You cannot expect them to use a condom every time."

If education programs continue to fail in the United States, how are they going to work in Latin America and the Caribbean, where there are 42 countries and half a dozen major languages with hundreds of dialects?

There are thousands of microcultures, from the Andean Indians to the denizens of the Amazon jungles to the English-speaking inhabitants of the Caribbean to the Creoles of Haiti. The cultures of Peru alone range from those with Japanese ancestry to blacks brought in as slaves to Spaniards to the descendents of the Incas.

"The fact that Mexicans and Argentines speak Spanish is not enough. It is not the same Spanish," St. John said. "Each country is going to have to develop its own message."

But can the same message of safe sex practices reach so diverse an audience?

Dr. Jonathan Mann, director of the World Health Organization's AIDS program based in Geneva, is hopeful. "We believe the spread of AIDS can be stopped," he said. But WHO leaders must maintain a precarious balance between sounding the alarm that a lethal disease is spreading and maintaining a professional optimism that profound changes can be made in different cultures.

While others are more doubtful that AIDS can be stopped, they hope the spread of the virus at least can be slowed to buy time for a vaccine and more effective drugs to be developed.

But changing sexual patterns in Latin America may be more difficult than in the U.S.

"Latins, in general, are not very open to talking about sex as we are in the U.S.," said Dr. Lydia Bond, director of PAHO's AIDS education program. "They do it, but they don't talk about it."

Latino men, in some regions, are actively bisexual but don't identify themselves as homosexual, Bond said. If the man is aggressive, macho, then he does not consider himself homosexual, even if he is having sex with another man. Only men who are dominated, who are passive, are homosexual, according to the Latin cultural definitions.

"In Latin America, homosexuals and bisexuals are not as open in the United States," Bond said. "They do not think of themselves as homosexual. Consequently, they are not easily found."

Prostitutes can be even harder to reach, Bond said. "In Latin America, if a prostitute is at risk of losing $10 for using a condom, she is not going to do it," Bond said. Compared to the risk of not being able to put food on the table for her family, the abstract risk of dying of AIDS sometime in the future is just too hard to contemplate.

In addition to cultural obstacles and poverty, the lack of medical services in most countries hampers public health campaigns.

One aggressive approach to stopping AIDS is under way in Cuba. More than one tenth of the 10 million inhabitants have been tested, as are any foreigners coming to live or Cubans returning from extended stays in other countries. Those nearly 150 Cubans known to be infected are essentially quarantined outside Havana.

Still, even in Cuba, there are loopholes in the program -- such as letting infected men lodged in the AIDS sanitorium to go home to their wives on weekends.

Pressed on whether AIDS can be stopped in Latin America and the Caribbean, St. John admits that "it's going to take a while."

But there is general agreement among the international medical community that every effort must be made to try to prevent its spread. "There are only two choices: do nothing or do something," St. John said. "Doing nothing is not acceptable. We know what will happen if we do nothing."