One more reason to worry.

In developing countries, one important, relatively affordable way of reducing HIV transmission may be to give HIV-infected women a dose of an antiviral drug during labor. However, a study of 15 women in Uganda suggests that a single dose of the AIDS drug nevirapine may prod the virus to develop drug resistance.

Six weeks after treatment with nevirapine, three women harbored HIV strains with a genetic mutation rendering them resistant to the drug. Blood samples taken from two of these women before delivery showed no evidence of the HIV mutations.

"We have enormous numbers of infants who are contracting and dying from HIV. This [new finding] should not take away from the success of this intervention," says Graziella Becker-Pergola of Johns Hopkins Medical Institutions in Baltimore. She and her colleagues plan to follow a larger number of women to see if the resistant virus passes from mother to child and whether over time the mutation conferring resistance fades from a person's viral population.