They said the Mississippi child is now 2½ years old and has been off HIV drugs for about a year with no signs of infection.
"We have, perhaps inadvertently, but in fact, cured the child," said Dr. Hannah Gay of the University of Mississippi Medical Center.
Doctors are still trying to piece together how it happened. The child was given three antiviral drugs within 30 hours of birth. A few months into treatment, doctors lost track of the mother. A year later, they discovered the mother stopped giving her child the medications.
"After the baby had missed several appointments, we started looking for her," Gay said.
They found the mother and her baby. The clinic ran tests to see how much HIV was in the baby's blood, and doctors found no trace of the virus.
"My first thought was to panic. I thought, 'Oh my goodness, I have been treating a child who's not actually infected,'" Gay said.
After lab work was checked and double checked, doctors from Boston and Baltimore were brought in to begin research.
The fast action of drug treatment apparently knocked out HIV in the baby's blood before it could form hideouts in the body. Experts say they hope this leads to eliminating HIV infection in children.
"This has very important implications for pediatric HIV infection and the ability to achieve cure, so we think we should be able to replicate this," said Dr. Deborah Persaud with Johns Hopkins' Children's Center Baltimore.
There has only one other case of someone being cured of HIV: a German man who received a bone marrow transplant.
The Associated Press contributed to this report.