The baby, born in rural Mississippi, was treated aggresively 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…

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.