Doctors have apparently cured a baby infected with H.I.V., the virus that causes AIDS, with an aggressive treatment of drugs starting a day after the baby was born. If confirmed by further analysis, this would be the first time a person has been cured with simple drug treatments, making a lifetime of antiviral therapy unnecessary.

Doctors can almost always prevent transmission of H.I.V. from infected mothers to their babies by testing the mothers during pregnancy, treating those found infected with antiretroviral drugs, and giving their babies a six-week course of one or two drugs to prevent them from becoming infected. That works in the vast majority of cases, but when it does not, doctors provide stronger drug regimens to treat the infection in the baby.

In the latest case, a Mississippi woman, who was infected with H.I.V., did not get treatment during her pregnancy. Doctors gave her baby a three-drug treatment-strength regimen, some 30 hours after birth. They continued the treatments for 18 months, after which the mother stopped going to the hospital and stopped giving her baby the drugs. Five months later, the mother returned with her child, who turned out to be free of the active virus. The baby, now 2 1/2 years old, has been free of the active virus ever since. Although very sophisticated tests can find traces of the virus, it is not able to replicate and spread. This is described as a “functional cure.”

There are reasons to treat this apparent breakthrough cautiously. Researchers must still demonstrate conclusively that the baby had truly been infected and was not simply prevented from absorbing its mother’s infection - a process achieved routinely in many babies. They must also show that this is not an exceptional, nonreplicable case with an atypical baby, but that the same treatment would work in other newborns.

If the stronger early treatment is confirmed to work, it could become the standard of care around the world. Fewer than 200 babies a year are born with lasting H.I.V. infections in the United States, but some 300,000 babies a year are born with such infections abroad. Even if the treatment works, the best defense against mother-to-child transmission is still prevention, which will always be preferable to treatment after an infection has set in.