“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 change the way newborns of infected mothers are treated all over the world. 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 3 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, giving a boost to research aimed at a cure, something that only a few years ago was thought to be virtually impossible. 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. “For pediatrics, this is our Timothy Brown,'’ said Dr. Deborah Persaud, associate professor at the Johns Hopkins Children’s Center and lead author of the report on the baby. “It’s proof of principle that we can cure H.I.V. infection if we can replicate this case.'’ Dr. Persaud and other researchers spoke in advance of a presentation of the findings on Monday at the Conference on Retroviruses and Opportunistic Infections in Atlanta. Some outside experts, who have not yet heard all the details, said they needed convincing that the baby had truly been infected. If not, this would be a case of prevention, something already done for babies born to infected mothers. “The one uncertainty is really definitive evidence that the child was indeed infected,” said Dr. Daniel R. Kuritzkes, chief of infectious diseases at Brigham and Women’s Hospital. Dr. Persaud and some other outside scientists said they were certain the baby – whose name and gender were not disclosed – had been infected. There were five positive tests in the baby’s first month of life – four for viral RNA and one for DNA. And once the treatment started, the virus levels in the baby’s blood declined in the pattern characteristic of infected patients. Dr. Persaud said there was also little doubt that the child experienced what she called a “functional cure.” Now 2½ , the child has been off drugs for a year with no sign of functioning virus.”