LUSAKA, ZAMBIA -- John Bwalya was supposed to sleep last fall with Alice, his brother's widow.

The Zambians call it "cleansing." When a man dies, his widow is expected to have sex with one of her in-laws, usually a brother. According to a widely held traditional belief, this rids her of the ghost of her husband and frees her to remarry.

Bwalya (who insisted on a pseudonym) says he was afraid to sleep with Alice, however, because his brother, after a year's illness, had died of acquired immune deficiency syndrome, or AIDS.

Despite pressure from the widow, her family and his own uncle, Bwalya adamantly refused to cleanse his sister-in-law. With the help of a sympathetic older brother, Bwalya fled his home village in northern Zambia's copper belt and moved here to the capital.

"It was like someone bringing you a coffin and saying you get in this coffin," says Bwalya.

The AIDS epidemic that has swept across Africa in the past five years has been exacerbated in Zambia by deeply entrenched tribal customs.

Those customs retain a strong hold on a large proportion of the country's 7 million people, in part because of officials' unwillingness to acknowledge publicly the large number of people who are infected with the disease.

That is the assessment of Zambian activists who say that the government intentionally under-reports the number of AIDS deaths in this southern African country.

Western observers here and international AIDS authorities hold a similar view.

"People in Zambia do not officially die of AIDS. When my cousin died of AIDS in March, we looked at his official death certificate and it said tuberculosis of the bones," said Emma Chibesakunda, chairman of the Catholic Women's League, an organization that is trying to publicize AIDS and challenge tribal customs that spread it.

Chibesakunda's willingness to speak openly about AIDS is a rarity here. The Ministry of Health last month imposed a gag order on all health workers in Zambia, ordering them not to release information on the disease.

Zambia officially acknowledges about 300 cases of AIDS and fewer than 100 deaths -- figures believed by international health authorities to be absurdly low.

Researchers at Lusaka's university teaching hospital last year found AIDS infection levels as high or higher than those reported anywhere in the world.

According to their findings, male patients between the ages of 20 and 30 were the most severely affected (38 percent). Pregnant women were the least affected (11 percent).

Of all patients who went to the hospital last year saying they were ill, 21 percent were infected with the AIDS virus, researchers found.

This figure is among the highest reported in Africa, which the World Health Organization says has the globe's most severe AIDS epidemic.

Unlike much of the world, where AIDS has been spread primarily through homosexual contact, intravenous drug use and blood transfusions, the disease in Africa is spread primarily through heterosexual contact.

Since the epidemic started in Africa in the late 1970s and early 1980s, men and women have contracted the disease in equal numbers.

Controlled case studies in Africa show that AIDS patients have had a significantly higher number of sexual partners than Africans who do not have the disease, an average of about 32 versus three.

"Even if it doesn't get any worse here, what has happened in Zambia is a tragedy of enormous proportions," said one westerner familiar with research data here.

Reticence about AIDS is more than just government policy in Zambia. It is consistent with a society where sex is treated with Victorian primness.

Censors cut kissing and hugging scenes out of movies. Sex-oriented magazines are banned. Public discussion of sex makes many people nervous, and an AIDS-related death is a source of profound family embarrassment.

John Bwalya, the young man who moved to Lusaka late last year to escape his brother's AIDS-infected widow, typifies his country's attitude about AIDS. He believes he and his family would be shamed if his real name were published in a newspaper.

The dilemma that Bwalya resolved by fleeing his home village is one faced by a growing number of Zambians, both men and women, as AIDS deaths increase.

"This cleansing custom is a living part of our culture," said Chibesakunda of the Catholic Women's League. "We believe that when your husband or wife dies, his or her spirit is with you. The disposing of the spirit can only be done through intimate contact with a relative of your spouse.

"Even very Christian women in this country believe they have no choice in this matter. You would be amazed."

Among the 73 tribal groups in Zambia, a landlocked country that is part of the so-called AIDS belt of central, eastern and southern Africa, only one tribe does not have a traditional belief in the cleansing ritual.

Some tribal groups do not insist on intimate sexual contact, allowing a number of substitute cleansing ceremonies that are morally acceptable to Christians.

The Kaonde tribe of northwest Zambia -- which includes Bwalya's sister-in-law -- is one of several that believes only sexual intercourse can cleanse a ghost.

"At the moment, because of Christianity and education, a certain group of people are beginning to believe that nothing bad will happen if they reject the ritual," said Chibesakunda. "But the majority believe they will be haunted and nobody wants to be haunted."

Traditional law in Zambia allows a widow to go to a local court in order to force one of her in-laws to sleep with her.

"Many women still view it as a privilege to be cleansed . . . But it is beginning to change," said Edith Mutale, an official of the Christian Council of Zambia and an activist for women's rights in the country. "This AIDS thing has now just come to amplify the things we are demanding as women.

"Women are asking, 'If I am AIDS free, must I die to go through this ritual?' "

The Christian Council of Zambia and the Catholic Women's League recently have sponsored a series of seminars around the country to raise awareness of AIDS and challenge ritual cleansing.

These efforts have provoked a backlash from some traditionalists.

Lily Mutale, a political leader in the Kapwepwe ward of Lusaka, was quoted in the Times of Zambia recently as saying that it was wrong for anyone to forsake culture for AIDS.

"AIDS or no AIDS, {cleansing} must take place," she reportedly said. "We shall be defeating our own culture if we sacrifice our tradition at the altar because of fear of AIDS."

Chibesakunda says that attitudes in Zambia's cities are likely to change faster than in rural areas, where cleansing rituals are taken more seriously. She adds that there is little likelihood that use of condoms will help.

"Zambian men don't believe in condoms and women think they hurt," she said.

Growing awareness of AIDS, however, appears to have revived another widespread tribal custom in the country, which may help check the spread of the disease.

Among the Bemba tribe, Zambia's largest, the custom is called chisungu. It demands that teen-agers refrain from sex until they are married. The custom, according to Zambians in Lusaka, had been relaxed in the past two decades with the country's rapid urbanization.

Having seen many young people die of AIDS, however, elderly women in rural areas recently have been demanding that chisungu be enforced.

"Zambians are paying dearly for going against the custom," said one 60-year-old woman from the village of Kaoma in western Zambia.

"In the past we told young people that if you sleep with a man out of wedlock, you will die a skinny person," said the woman, who insisted that her name not be used.