HIV positive mothers who breast-feed run a 28 percent risk of transmitting the virus to their infants, says a new study from the Baragwanath Hospital in Johannesburg, South Africa. Earlier research put the likelihood at 14 percent, according to the Centers for Disease Control and Prevention. These findings are especially grim for moms in impoverished countries, who have been continually urged to breast-feed even if they have HIV.

In 1992, the United Nations' Children's Fund and the World Health Organization (WHO) concluded that diarrhea and malnutrition pose a far greater threat than AIDS to infants in developing nations, where formula is expensive and clean water scarce.

"It's a really tragic situation. you're talking about HIV transmission to some 30 percent of infants. Yet if you don't breast-feed, the morbidity and mortality is even more horrendous," said Dr. Katherine Luzuriaga, a pediatric AIDS researcher at the University of Massachusetts Medical School.

Researchers are studying the possibility of administering antiretrovirals to stem mother-to-infant transmission. In the meantime, the United Nations AIDS Program says mothers should be advised of the risks and make their own decisions. WHO spokesperson Celinda Veramo said the agency, which sponsored the first annual "World Breast-Feeding Week" last August, had no plans to change its advisory that "all babies should be fed exclusively on breast milk until six months of age."