Doctors announced on Sunday that a baby had been cured of an H.I.V. infection for the first time, a startling development that could change how infected newborns are treated and sharply reduce the number of children living with the virus that causes AIDS.

The baby, born in rural Mississippi, was treated aggressively with antiretroviral drugs starting around 30 hours after birth, something that is not usually done. If further study shows this works in other babies, it will almost certainly be recommended globally. The United Nations estimates that 330,000 babies were newly infected in 2011, the most recent year for which there is data, and that more than three million children globally are living with H.I.V.

If the report is confirmed, the child born in Mississippi would be only the second well-documented case of a cure in the world. That could give a lift to research aimed at a cure, something that only a few years ago was thought to be virtually impossible, though some experts said the findings in the baby would probably not be relevant to adults.

The first person cured was Timothy Brown, known as the Berlin patient, a middle-aged man with leukemia who received a bone-marrow transplant from a donor genetically resistant to H.I.V. infection.