PORT-AU-PRINCE, HAITI -- The nuns rustle through the wards of Mother Teresa's Home for the Dying here, barely defying the almost unnatural hush that blankets the place like a shroud. In the upstairs ward, a dozen women lie crumpled on beds throughout the room, most of them skeletal, squinting at visitors, too weak to sit up. The scene downstairs in the men's ward isn't much better. One man is moaning raspily; another has vomited and is writhing silently with cramps.

Many of these patients will not live out the month, and some won't know what killed them. "What illness do you have?" they are asked one by one. "Have you been to see a doctor? Has your blood been tested?"

A few say they have tuberculosis. To most, the question of diagnosis seems beyond the point, not a matter of very great interest. Again and again they reply simply that they are malade -- sick -- that they don't know what illness they have, and that it seems to be getting worse.

But Sister Kamal, the young Indian nun who was recently appointed Superior of the Home, has a good idea what illness has led these patients here. More than half of them have AIDS, she says, judging by the symptoms and a sampling of blood tests. What's more, she says, only a slightly higher proportion of the men than women appear to be suffering from the disease.

The worst fears that haunt many Americans about AIDS -- that the disease will spread through the heterosexual population, infecting women and men in roughly the same numbers -- are already coming true in this impoverished Caribbean nation 600 miles from the Florida coast.

Today, heterosexual intercourse in Haiti accounts for more new cases of AIDS and a less severe disorder, AIDS-related complex, than all other factors combined. Four out of five men and three out of five women who are infected with the disease this year will get it from contact with a partner of the

opposite sex, researchers say. More than 40 percent of the conservatively estimated 1,500 AIDS patients in Haiti are women.

While the overall rate of AIDS in Haiti is not much higher than in the United States (and may be lower than in the cities of New York and San Francisco), the statistics documenting AIDS' spread to heterosexuals represent a stunning reversal from the pattern of the disease just four years ago. At that time, the profile of AIDS in Haiti resembled that of AIDS in the United States: Most Haitians infected with the AIDS virus were homosexual or bisexual men or recipients of transfusions and blood products.

But now the spread of AIDS in Haiti resembles more closely the situation in Africa, where the disease also appears to be spread primarily through heterosexual contact.

This evolution of the disease in Haiti, and the speed with which it took hold among heterosexuals, has prompted researchers to examine the underlying causes of the shift -- and to ask the critical question: Will it happen in the United States? On Harry Truman Boulevard, running along the seaside in Port-au-Prince, in a low-slung, formerly abandoned building marked "Institut National de Laboratoire et de Recherches," Haiti's leading AIDS re-

searcher toils in a cubicle that seems barely large enough to contain his tall frame.

A deeply serious man with a trim beard and spectacles, Dr. Jean William Pape returned to Haiti in 1980 after studying medicine at Cornell University. Initially, his plan was to work in the fields of infantile diarrhea and oral rehydration. That plan didn't last long.

"We started seeing a number of cases of incurable diarrhea in adults," he said in an interview recently. "At first, I thought it was TB of the intestines -- which we can cure. But that wasn't the case. This was very confusing."

In the ensuing year, Pape (pronounced Pop), in concert with several other Haitian doctors who had also seen a succession of puzzling afflictions, established Haiti's only AIDS clinic, with U.S. grants from Cornell and the National Institutes of Health.

The thrust of Pape's research has been to determine how AIDS is spread in Haiti. In the course of his sleuthing, he has discovered that some of Haiti's most deeply ingrained social patterns -- patterns of sexuality, poverty and health care -- may be largely to blame for the surprise attack that AIDS has made on Haitian heterosexuals.

Pape and other Haitian researchers think that AIDS was introduced to Haiti by American and Canadian homosexuals who vacationed in the West Indian nation in the late-1970s.

Although homosexuality is scowled upon here -- most Haitians say it does not exist in their country -- some people assented to having sex with tourists because it was profitable, according to the researchers. In Haiti, the poorest country in the Western Hemisphere, the profit motive is powerful.

"There are two groups of homosexuals," said Dr. Bernard Liautaud, the dermatologist who diagnosed Haiti's first case of AIDS-related infection in 1979. "There are those who do it for pleasure and those who do it for economic reasons. In Haiti, we have economic homosexuals -- poor people making love for money."

Haiti's "economic homosexuals" were in fact bisexuals, many of them married or living with female partners, the researchers say. As a result, the virus that arrived with homosexual tourists quickly spread to Haitian women, according to the researchers.

If that theory is correct, it would be a significant departure from the spread of the disease in the United States, where homosexuals have passed the virus largely within their own, sexually distinct community.

In Haiti, after the disease got a foothold in the general population, its spread was assured by what the researchers characterize as a pattern of promiscuity that is common among Haitian men. While more than 90 percent of Haitian women interviewed by AIDS researchers report having a single sex partner, most of the men say they have several.

Such men are not necessarily rich. Simone Bolivar, who has not held a steady job in three years, scrapes by on the cash he can beg and earn as a guide from tourists in Port-au-Prince. He has two wives, one in the city and one about 50 miles outside town, and a child by each of them. "Many, many Haitians do this," said the 24-year-old Bolivar. "We like having more than one woman."

Additionally, Bolivar said, he often visits the seedy Carrefour section of town, where hundreds of Haitian and Dominican female prostitutes work the bars, nightclubs and streets. According to Haitian AIDS researchers, between 50 percent and 80 percent of these women are infected with the AIDS virus.

Ira P. Lowenthal, an American anthropologist who has studied and lived in Haiti, wrote that Haitian men "may simultaneously marry, maintain a consensual wife in a second household, and conduct one or more relatively stable extra-residential affairs."

To Pape, that anthropological finding is a fact of daily life. "Look at this," he said one morning early this month, waving a questionnaire that had been filled out that day by an AIDS patient labeled No. 1606, a Port-au-Prince salesman. "In a four-year period, from 1982 to 1986, he says he had 20 sex partners. This is a little more than most, but not much."

The man also reported having had sex with prostitutes 100 times. Moreover, like 95 percent of men who come to Pape's clinic in this heavily Catholic country, he said he did not use condoms.

"This is a tragedy," said Pape. "The education really has to start with the males, because this is who is spreading the disease in Haiti." Folk Healers -- With Needles

Tiny holes, no more than white specks in the corrugated tin roof over Viviane Vilbruin's head, admit slanting rays of light into her sitting room, where a dozen kids are crouched in front of the television.

The TV had blinked off a few minutes earlier -- some sections of this city lose power frequently -- but it would take a while for the children to give up and wander out of Madame Vilbruin's place: she owns one of the few televisions in the neighborhood.

A kindly, modest woman with a warm smile that reveals her two teeth, Vilbruin is a pillar of her community, a poor section of Port-au-Prince called Saint Antoine. In addition to providing the closest thing to a cinema that many of the neighborhood kids have ever seen, she is a tutor, a tailor, a dispenser of wisdom and of gossip.

Among the other services to the community she renders, Vilbruin is known far and wide as a piqurist -- one of perhaps 10,000 untrained, uncertified but widely trusted givers of intramuscular injections who operate freely in Haiti. AIDS researchers and public health officials say that piqurists, who dispense penicillin, vitamins and other drugs on demand -- sometimes use needles repeatedly without sterilizing them -- a practice that Vilbruin says she does not follow.

"I've heard of others who give injections without cleaning the needle," she says in her native Creole language. "But not me, I always do it."

Vilbruin says she has heard about "SIDA" -- the French acronym for AIDS -- and understands the importance of using sterile, disposable needles for injections.

Piqurists such as Vilbruin are popular in Haiti; nearly every town or neighborhood has one. Health officials say the lay injectionists are a valuable supplement to the health care system of a nation where doctors are sometimes in short supply.

"There's a saying here that injections are better than pills -- that it has got to hurt to work," said Pape. "And so the piqurists are popular."

Haiti has no significant number of heroin addicts or other intravenous drug users, but the wide acceptance of injectionists as health practitioners is seen as a factor in the spread of AIDS through the general population. Researchers estimate that perhaps 5 or 6 percent of the AIDS cases in Haiti are transmitted through the use of contaminated needles.

"We're thinking about recruiting the piqurists and informing them of the dangers," said Pape. "We need them."

Another problem in Haiti is the frequent use of blood transfusions. Pape and other AIDS researchers are trying to discourage Haitian doctors from the relatively common practice of giving blood transfusions to women who have just given birth. An estimated one fourth of all female AIDS patients in Haiti were transfused with blood that contained the AIDS virus. Doctors there say the Red Cross started screening blood supplies in Port-au-Prince only last year.

Blood supplies in rural areas, where 80 percent of Haitians live -- and where the extent of AIDS infection is unknown -- remain unscreened. While few cases of AIDS have been reported in the provinces, doctors are worried that because of spotty rural health care and poor communication links with the capital, the virus may be spreading in the countryside without their knowledge.

Mother to Child

Pape, Liautaud and the other doctors who have organized the AIDS clinic in Port-au-Prince seem committed to explaining the virus and the disease to patients, their patients' partners -- anyone who may be at risk. Every week, they warn of the dangers of transmission and dispense hundreds of condoms.

But there are no fixed rules concerning the treatment of AIDS in Haiti, and at least one doctor has decided not to tell infected patients that they carry the AIDS virus.

A year ago, Dr. Reginald Boulos launched what he thought would be a study of prenatal care, premature births and infant mortality. With grants from the National Institutes of Health, the U.S. Agency for International Development and local funds, he selected 3,000 pregnant women in Cite' Soleil, a part of the city where perhaps 150,000 people are wedged into four square miles of crumbling huts, open sewers and unpaved streets.

In routine blood tests at the outset of the study, Boulos found that 8 percent of the pregnant women, or about 250 of them, carried the AIDS virus. Of those, only a relative handful had developed the disease. Suddenly, Boulos realized he had a different study than the one he conceived. In addition to studying prenatal care, he decided to track the virus: In what cases would it be transmitted to the babies? What role would it play in infant mortality?

He also made a decision not to inform the infected women, at least not yet.

"Right now, we can't tell them," he said in an interview. "It's hard for someone in Cite' Soleil -- someone who has never been to college, who's illiterate -- to understand they're infected but not sick. We wouldn't want to break up marriages. We wouldn't want to turn people against us.

"An American might tell their girlfriend, or their boyfriend, because they'd understand. But these people would not understand . . . They didn't come here to be tested for AIDS. And to dump information on people, this is inhumane. I'm not going to tell them, 'You have the virus and there is nothing I can do for you.' If you go today and tell them, you're going to lose them. People who are not sick do not want to know they are AIDS victims."

Boulos is hoping to open a clinic for sexually transmitted diseases in Cite' Soleil at the end of this year. Then, on a case-by-case basis, some of the infected women may be informed -- if social workers think they can digest the information, according to Boulos. In the meantime, he acknowledged, "maybe we'll risk a few people."

An American-trained physician, Boulos has worked in Cite' Soleil for several years, leading a public health drive there that has earned him the respect of doctors throughout Haiti. Infant mortality in Cite' Soleil has dropped by more than 60 percent in the past decade, a shift that Boulos cites with pride.

Still, Boulos' ongoing study raises difficult ethical questions, and his decision not to inform patients about the AIDS virus does not sit well with other AIDS researchers in Haiti. Pape called it "morally wrong" and insisted that the women could understand the infection if it were explained to them simply. "They need to warn their sex partners," he said.

It is unlikely, however, that any agency of the government will intervene in the study or get involved in setting guidelines for the treatment of AIDS patients and others infected with the virus.

At one point, a national commission on AIDS did start to devise a set of ethical guidelines for the treatment and counseling of AIDS patients. But midway through drafting the guidelines, Pape said, there was a change in health ministers: political problems. The project, like so much in Haiti, seems to have fallen by the wayside.

Crystal Ball for the U.S.?

In the United States, fewer than 5 percent of AIDS cases have been traced to heterosexual contact. The large majority of people with the disease in the United States are still homosexual men (66 percent) or intravenous drug users (16 percent). Another 8 percent of the U.S. cases are in homosexual i.v. drug abusers. At the same time, concern has grown steadily in recent months that the virus may be spreading undetected among heterosexuals.

Because there is frequently a five-year lag between infection with the virus and the appearance of symptoms, the extent to which AIDS is already present in America's heterosexual population is not yet clear.

Studies performed on U.S. military recruits indicate that heterosexual men and women are being infected at about the same low rate, but that rate appears to be holding steady, not increasing as would be expected if the disease were spreading rapidly in the general population. The military data, moreover, is not representative of the entire U.S. population since those tested predominantly are young and disproportionately members of minority groups.

Other studies have shown that the AIDS virus is spread heterosexually among the sexual partners of intravenous drug users who became infected from contaminated needles.

The general good health of American heterosexuals may also prove to slow the spread of the virus. Many scientists believe that co-factors -- other sexually transmitted diseases -- increase the chances of infection. They fear that gonorrhea, syphilis or other STDs may break down the protective linings of the genital tracts -- linings that may block the virus -- allowing the virus to get in. Those STDs tend to be common in parts of the world where AIDS is widely transmitted through heterosexual contact, such as Africa and Haiti.

Still, there is much uncertainty about the future spread of the disease.

"Nothing is predestined, either in Haiti or in this country," said Dr. Warren D. Johnson Jr. of Cornell University Medical College in New York, who leads the American side of a Haitian-American AIDS research team. "What the future holds depends on how successful we are at educating people on how to avoid {AIDS}."

Nonetheless, some Haitian researchers are grimly confident that AIDS will evolve in the United States as it has in Haiti -- more slowly, to be sure, but with devastating effect, eventually spreading among heterosexuals in large numbers, widening its swath of suffering and sorrow.

"It will come slower in the U.S., but it will come," said Dr. Bernard Liautaud, a Port-au-Prince dermatologist and AIDS researcher. "Really, I am quite sure of that."