AIDS is galloping across Africa, killing up to a third of the adult population in some areas and leaving thousands of orphans, including many infected with the virus, researchers said Saturday.

Projections by the U.S. Bureau of the Census forecast that by 2015 there will be more than 70 million cases of AIDS in the countries south of the Sahara Desert.AIDS-related infections already represent up to 80 percent of the hospital admissions in Zambia, said Dr. Peter H. Perine of the Uniformed Services University of the Health Services.

"It has devastated Zambia," he told the American Association for the Advancement of Science. "It is overwhelming the health-care system in the country."

About 22 percent of the women of reproductive age in the Zambian capital are infected with the human immunodeficiency virus, or HIV, that causes AIDS, Perine said. "It's likely to be as high as that in men."

Peter Way of the Census Bureau said, based on estimates by his agency, AIDS would be the major cause of death among adults in African countries south of the Sahara by 2015. "Some areas already have that level of infection," he said.

Forty percent of the adult population in some cities in Tanzania is infected, Way said, adding that the infection rate is 30.3 percent in Rwanda's capital of Kigalli.

Linda A. Valleroy of the Agency for International Development said that in Kampala, Uganda, AIDS is expected by 1992 to almost double the rate of death - to 53 per thousand - among women aged 25 to 35. A similar AIDS death rate increase is expected among adult males.

Because most of the AIDS-related deaths are among able-bodied men and women who are rearing families, many children are left without parents.

"By 1992, we estimate that the mortality of women of a reproductive age will be doubling and the number of orphans will be doubling," she said.

Valleroy said there are more than 250,000 orphans in sub-Saharan Africa as a result of AIDS and the number may rise to 16 million by 2015.