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.