For the third time in recent years, a child born with H.I.V. has been found free of the virus for a long period after a high dose of treatment early in life.

The discovery has raised anew the hope that early treatment, in some cases, may allow an infected person’s immune system to defeat the virus. But proof has remained elusive, and there have been more disappointments than triumphs.

The child — a 9-year-old South African girl — has been in remission for over eight years, according to research presented Monday at an International AIDS Society conference in Paris.

The case “strengthens our hope that by treating HIV-infected children for a brief period beginning in infancy, we may be able to spare them the burden of lifelong therapy,” Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a description of the case released by the institute, which sponsored a clinical trial in which the child was enrolled.