Belgian medical researchers are reporting an alarming increase in the prevalence of the fatal disease AIDS in some African nations where, in contrast with the United States, it is spreading chiefly among the heterosexual population.

The African nations where the disease has been spotted are Zaire, Rwanda, Burundi, Kenya, Tanzania and Uganda.

Researchers believe the disease could spread from equatorial to West Africa and eventually pose a much larger public health problem than in the United States or Europe, due in part to its being established among heterosexuals and the prevalence of such other diseases as malaria that seem to make Africans more susceptible to AIDS.

AIDS, or acquired immune deficiency syndrome, destroys the body's natural immunities to other diseases. It is nearly always fatal. In the United States its largest incidence has been among homosexual males, Haitians, intravenous-drug users and recipients of donor blood.

According to Dr. Nathan Clumeck, head of the Division of Infectious Diseases at Brussels' St. Pierre Hospital, the extent of AIDS in some African countries is already alarming. In Rwanda, for example, surveys have shown that about 10 percent to 18 percent of the adult population has been exposed to the AIDS virus.

In Zaire's capital, Kinshasa, with a population of more than 3 million, about 26 out of every 100,000 residents is thought to have AIDS. Doctors at the city's main public hospital are seeing 10 to 20 new cases of AIDS each week, Clumeck said.

The Belgians' familiarity with the health problems of Africa has its origins in their country's former colonial role on the continent. Zaire was formerly the Belgian Congo; Rwanda and Burundi were once Belgian trust territories under League of Nations and United Nations auspices. Medical workers here began tracking AIDS in Africa when victims of the disease from the continent were admitted to Belgian hospitals earlier this decade.

The growing threat of AIDS leaves African health authorities, who are facing limited budgets and growing challenges from traditional tropical diseases and other maladies, with a difficult choice when considering where to concentrate their efforts.

"The authorities have to determine whether AIDS is a priority," said Dr. Peter Piot, head of the department of microbiology at the Institute of Tropical Medicine in Antwerp.

"I'm not so sure that it is, when you have thousands of people dying from tuberculosis, malaria, measles and malnutrition," he said. Rwanda, for example, has the equivalent of about $1 to spend yearly per person on health programs. "You can forget about screening blood donors for the AIDS virus there," he said.

In their studies of African AIDS victims, the Belgian researchers have discovered that, as in the United States, sexual promiscuity plays a strong role in the rapid spread of the disease. Of 58 male patients with AIDS or the related complex known as ARC who were examined in one study, 81 percent had regular contacts with prostitutes and had an average of 32 sexual partners per year.

About 80 percent of the prostitutes surveyed in Rwanda were found to have been exposed to the AIDS virus, which Clumeck said was the same rate as among homosexuals in San Francisco.

Why has AIDS exclusively afflicted heterosexuals in Africa rather than homosexuals? The simplest answer is that homosexuality is rare in the countries affected.

But the AIDS epidemic has also been facilitated by the changing social conditions in African cities, where rapid growth and exposure to western culture have broken traditional patterns of life, Clumeck said.

Women who cannot find work in the cities turn to prostitution, for which there is a growing demand.

"For rich people, polygamy is not allowed, but rich people want to show to others that they are wealthy, and for that there are many, many women," Clumeck said. The African AIDS patients studied in Brussels and those in Zaire and Rwanda were mostly from the professional classes and had well-to-do backgrounds, he said.

Other African factors the researchers said might encourage transmission include unhygienic practices such as the reuse of hypodermic needles. Malaria and other tropical diseases are suspected of encouraging the development of AIDS, a role the cytomegalovirus is thought to perhaps be playing in the United States and Europe.

Mosquitoes have been ruled out up to now as transmitters of the disease in Africa, because it has affected only the sexually active adult population. If enough people are infected, however, insects could play a part in the more rapid spread of AIDS, Clumeck said.

Although some evidence has pointed to the African green monkey as the possible source of the AIDS virus, the Belgian researchers are divided over the question of whether the disease originated on the continent.

Clumeck said he believed AIDS first developed in Africa and spread to the United States, then to Europe. The apparent first cases of AIDS in Africa can now be identified as far back as 1973, he said. Microbiologist Piot, who spends about two months a year doing field research in Africa, said he believed the African records were not sufficient to make such a claim with any degree of certainty.

Piot also noted that at first homosexuals had been blamed for AIDS, then Haitians; he said it was unfortunate and perhaps unjustified that Africans would now suffer from the accusation.

Given the seriousness of the disease, he said, "we have to be careful we don't stigmatize a new group. From a scientific point of view, we don't have evidence that AIDS originated from one part of Africa. There are only lots of hypotheses." Relations between whites and blacks in some European countries, including Belgium, are bad enough, he said, without adding a further source of irritation.

The notoriety associated with AIDS has led at least one African country, the Congo Republic, which neighbors Zaire, to discourage efforts to measure the extent of the disease inside the country, the researchers said.

"Most countries don't like to be confronted with a disease like AIDS," one Belgian doctor said. The United States "doesn't like it when we say that AIDS was imported from America to Europe. The politics of this kind of disease are not confined to Africa."

The researchers agree, however, that AIDS is most likely a new disease in Africa. They point out that new, fast-growing versions of one form of cancer associated with AIDS, Kaposi's sarcoma, have been showing up with greater frequency, as have opportunistic infections linked to AIDS.

Because of the specific African factors that encourage the transmission of AIDS, Clumeck said, the chance that it will eventually spread rapidly among heterosexuals in western countries was less than the threat the disease posed to other African nations.