NEW YORK — Doctors announced Sunday that a baby had been cured of an HIV infection for the first time, a startling development that could lead to more aggressive treatment of babies infected at birth and a sharp reduction in the number of children living with the virus that causes AIDS.

The baby, born in rural Mississippi, was treated 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.