PORT-AU-PRINCE, HAITI -- In Crib 19 at St. Catherine's Hospital in one of this city's worst slums, Christella Joseph lies dying.

At 3 months old, Christella weighs just over two pounds. Her mother died a month after she was born, and no one is quite sure what happened to her father.

Christella is so dehydrated that when you pinch her delicate skin, it stays creased like paper. Her hands and feet are impossibly bony, something like a chicken's. With no one to care for her, she sucks and chews relentlessly on the knuckles of her left hand.

Christella was born with AIDS, the disease that probably killed her mother. The prognosis, said Joseph Fequiere, St. Catherine's director, "is very poor -- we don't expect her to live. . . . Actually it's incredible she's still here. She has no {parents} to take care of her."

In the other 40 cribs of St. Catherine's pediatric ward, Fequiere, a physician, guesses that four or five other children have the HIV virus associated with AIDS. For infants younger than a year old, AIDS testing is so often unreliable that it is of dubious value, and doctors are left to make educated guesses based on babies' symptoms. In Christella's case, the evidence is simply overwhelming.

In hospitals and slums all around this sweltering capital, babies afflicted with diarrhea, malnutrition and dehydration can be found by the thousands. Government and private estimate indicate 1,000 Haitian babies will be born infected with the HIV virus this year, most of them in Port-au-Prince.

To many health professionals, who for years have fought to whittle away at Haiti's once-staggering infant mortality rate, the question of determining which babies have AIDS and which do not is largely academic. Because AIDS breaks down the body's immune system and leaves its victims prey to a host of maladies, its symptoms are often those of the diseases it brings on.

But there is mounting evidence that the disease's toll among babies is rising, even as public-health workers point to preliminary signs that the overall rate of AIDS in Haiti may be leveling off. Perhaps most discouraging are studies showing that, after years of steady gains in keeping babies alive, some of the progress is being reversed.

Reginald Boulos, a Haitian AIDS expert and researcher, has tracked infant deaths in Cite Soleil, the slum where St. Catherine's is located. In 1976, Boulos said, the mortality rate for babies there was 236 per 1,000 -- nearly one baby in four.

Thanks to a concerted public-health campaign in Cite Soleil led by Boulos and other privately funded health workers, the rate fell by 1982 to 84 per 1,000. Since 1982, however, as the AIDS scourge has spread in Haiti, particularly in cramped urban slums, the mortality rate for infants in Cite Soleil has crept up again, to about 120 per 1,000, according to Boulos. He said he believes that at least half of the increase can be explained by AIDS.

"It's not wiping out all the gains we have made, but we are seeing a reversal," said Boulos.

Although Cite Soleil, a slum of about 150,000 people, has a better health-care network than most areas of the country, it also has a higher-than-average prevalence of AIDS. Between 8 and 10 percent of sexually active adults have the disease in Cite Soleil, compared to 6 to 7 percent of such adults throughout the country.

Boulos and other health researchers pointed to Haiti's seemingly chronic political instability as a factor in the rising infant mortality toll. The country has had six governments and five presidents in five years, none of them freely elected.

"Each time there is a coup, a large number of people leave the city, at least temporarily, and go back to the countryside, where there is no good health care," said Boulos. Only about 2 in 5 Haitians have access to adequate health care, he said.

The problems of combating AIDS in Haiti, where it is known as "death disease," remain daunting. While nearly everyone has heard of AIDS and knows that it kills, only about 40 percent of Haitians are thought to know how it is transmitted.

Sexual promiscuity is still widespread, with many men visiting prostitutes in Port-au-Prince, more than half of whom are thought to be infected. Nearly as many women as men have the disease, and heterosexual transmission is common. With the economy in a state of total collapse, many women who are not prostitutes take sexual partners who can provide them and their children with even a meager source of income.

"If we want to reduce the transmission of AIDS in Haiti, we need to improve the economy in Haiti," said Gilles Poumerol, a French epidemiologist for the Pan American Health Organization who works with the government's AIDS program.

International AIDS researchers have found that between 25 percent and 30 percent of mothers infected with the HIV virus pass it on to their newborn babies. It is not known why some mothers transmit the disease while most do not, but the risk appears to be higher in cases in which the disease has progressed to an advanced stage in the mother.

In the United States and other developed countries, mothers with the virus are advised not to breast-feed their infants, even though the evidence of transmission through breast milk is sketchy.

In Haiti and other poor nations, however, doctors advise mothers with the virus to breast-feed their babies. The risks of not breast-feeding in the Third World, including impure water and unreliable supplies or misuse of infant formula, are thought to be greater than the chances of transmitting the virus through breast milk.

Even for the babies who are not infected with their mothers' viruses, the threat to long-term health is considerable. If the mothers develop AIDS and die, the babies are often left untended. In a country where many children develop diarrhea on the average of once a month, the absence of a mother's care can be devastating.

"The child in Cite Soleil or {the northern city of} Gonaives who loses a mother or father {to AIDS} may not die of AIDS himself but ultimately will die for lack of care -- from {tuberculosis} or diarrhea or whatever," said Boulos.