RIO DE JANEIRO -- As the parades of Carnival pass by and the costume balls wind down amidst sequins and confetti, leaflets carpet the streets warning revelers who "play around" to "play safely." The reference is to a slow-motion disaster in Brazil -- the spread of AIDS.

While AIDS is in greater concentration in some parts of Africa, the number of official reported cases in Brazil is second only to that in the United States.

Authorities are fighting AIDS with a critically inadequate public health system that so far has been unable to convey even the most basic messages about prevention and treatment. They are trying to reach a population that is little educated, desperately poor and confronted with other health problems -- tuberculosis, for example -- that seem more pressing.

The leveling off seen in some other countries has not reached AIDS in Brazil. According to official figures, Brazil has seen about 15,000 cases, but some experts estimate that because of underreporting the true number may be three times that high.

More alarming is the rate of infection with the HIV virus that causes Acquired Immune Deficiency Syndrome, which is generally spread through sexual contact or blood. A study under way by researchers at the hospital of the Federal University of Rio de Janeiro has tentatively shown an HIV infection rate at least four times as high as expected -- this in a segment of the population, pregnant women, that was not previously thought to be at high risk.

Since the virus is believed to incubate for years before causing disease, the true crisis is yet to come. Estimates of the number of Brazilians already carrying the virus range from 500,000 to 1.5 million.

"Any way you look at it, it's frightening," said Dr. Mauro Schechter, who is coordinating the study. "The health system doesn't have the capacity for this. We were overstretched even without AIDS."

Public health authorities throughout Latin America have begun warning that AIDS is becoming a major problem. Researchers in Argentina, for example, announced last week that they have detected a sharp rise in HIV infection in Buenos Aires. But nowhere is the alarm greater than in Brazil.

The problem is most severe in the mega-cities of Sao Paulo and Rio de Janiero, where it has hit hardest among the same groups that have been at highest risk in the United States -- homosexual and bisexual men, intravenous drug users and their sexual partners, and hemophiliacs and others who receive blood transfusions.

But authorities are also worried about the potential spread of the disease in other urban areas, and especially in the rough-and-tumble settlements of Brazil's interior, where gold miners, construction crews, truck drivers and prostitutes mingle.

President Fernando Collor de Mello, like his predecessor Jose Sarney, has launched campaigns to make Brazilians aware of the danger. But critics charge that the government has primarily succeeded in frightening people without giving them simple, concrete information that could help protect them. Further, they fear that potential victims may be stigmatized, contributing further to the difficulties of control.

The most recent national effort is a campaign of posters, billboards and ads featuring outline drawings of a male figure and a female figure with bulls-eye targets drawn over their genital areas. Above is the campaign's slogan: "If you don't take care, AIDS is going to get you."

The posters state in smaller print that AIDS is usually transmitted through sexual contact, but say nothing about preventive measures like condom use and practicing what has become known as "safer sex."

"The campaign is a warning . . . {but} who is being warned?" wrote Herbert Daniel, an author who heads the Brazilian Interdisciplinary AIDS Association, a lobbying and support group. "What exactly is being said?"

The Roman Catholic Church -- Brazil has more people who identify themselves as Catholics than any other country -- has played a dual role. In the Sao Paulo area, where the church has a history of social activism, some priests have worked with AIDS groups in educational campaigns. But some other prominent clerics have hewn much closer to the Vatican's official position against all forms of artificial contraception, including condoms.

"The diffusion of condoms promotes sexual permissiveness and makes the situation worse," Bishop Boaventura Kloppenburg wrote last week in the Rio de Janeiro daily Jornal do Brasil. "Spiritual immunodeficiency leads to physical immunodeficiency."

Some private groups have tried to fill the gap. The "play safely" leaflets that litter the esplanade along Copacabana beach, for example, are put out by a gay support group called ATOBA and feature specific "safer sex" guidelines and a graphic illustration of how to use a condom properly. But most AIDS activists say that the federal government has yet to launch an educational campaign that has any chance of reaching people at risk.

"What we get instead is a kind of terrorism," said Herbert de Souza, a sociologist who focuses on AIDS issues. "They frighten people, but they don't inform."

De Souza's interest in AIDS grew out of circumstance, He is a hemophiliac who learned in 1985 that he had been infected with the HIV virus, probably through one of the frequent blood transfusions his medical condition requires. He has watched as his two brothers Henfil and Francisco, also hemophiliacs, died of AIDS.

The death three years ago of Henfil, who was a famous cartoonist, focused public attention on the AIDS risk posed by lax controls over the blood supply.

At the time, nearly 10 percent of Brazilians with AIDS had contracted the virus through blood transfusions, while the comparable figure in the United States and other countries was much lower. Brazil was several years behind in applying measures to screen the blood supply, and even now AIDS activists say there is a higher risk from blood here than elsewhere.

Herbert de Souza, 55, is so far free of any symptoms of disease. He sees a private doctor and is on the virus-inhibiting drug AZT, which costs him hundreds of dollars a month -- measures far beyond the reach of the vast majority of Brazilians.

"The very rich who have AIDS can go to private clinics, which can cost as much as $800 a day," de Souza said. "Everybody else has to go to the public hospitals. But there's no space."

Schechter said the hospital where he coordinates AIDS research has the largest public AIDS treatment program in Rio, having seen more than 1,000 patients thus far, or just under half of the reported total for the city.

At any given moment the hospital has beds for only about 25 people with AIDS. An AIDS outpatient clinic sees around 40 patients a day, and the hospital also runs what Schechter said is the only dental clinic for HIV-infected people in the country.

"This hospital was opened in 1978, and there have been no more public hospitals opened in Rio since then," he said. "But the number of public hospital beds has been going steadily down -- hospitals have been closing wards. Meanwhile, the economic situation is so bad that people who were previously able to afford private care are forced into the public health system."

Brazil has been caught in a downward spiral of inflation, stagnation and debt, with the result that the middle class is being squeezed. This country of more than 140 million people has one of the most polarized income distributions in the world.

The popular perception is that AIDS is a disease of the underclass. One prominent political scientist said recently that "there's nothing we can do for the poor, but why worry? They'll all die of AIDS anyway."

"You can't remove AIDS from the whole picture of the country," Schechter said. "Although I am very involved in AIDS research, I don't think it is the most important problem facing the Brazilian people. But it ought to be possible to do something about AIDS a lot more easily than doing what really has to be done, which is to change the whole economic situation of the country and do away with all the diseases of poverty."

Schechter's study is testing blood samples from 5,000 pregnant women who have come to Rio public hospitals to give birth. With around 4,000 samples already examined, researchers have been stunned at seeing an infection rate four times higher than expected.

Most infected women in Brazil were believed to have been exposed to the HIV virus through intravenous drug use or being the sexual partners of intravenous drug users. But Schechter said the study suggests either that women are contracting the disease through sexual contacts that were not previously thought to be risky, or that there are many more intravenous drug users in the city than authorities had imagined.

"First we need to do more work to find out just who the people are who are at risk," Schechter said, "and then we need educational campaigns that will reach them. . . . The campaigns so far have just created a new kind of leper. One of the campaigns on television had a man saying, 'I have AIDS, and there's no cure.' One of my patients tried to commit suicide after seeing that ad."

The stigma of AIDS -- the feeling that if the disease is bad, then the people who have it are bad -- is seen here as a major factor in the failure of prevention efforts to date. People who might be at risk are inclined to put the disease, and the things they might do to keep from getting it, out of their minds.

"One of the most important things to me is not to be infected with the idea of being sick," said de Souza. "I am not a virus. I am a man who will die, like everybody else."