A child thought cured of HIV by aggressive drug treatments immediately after her birth -- a result raised hoped of an eventual AIDS cure -- has relapsed and is showing signs of the virus, doctors say.

The child, born in Mississippi to an HIV positive woman who had received no treatments for the condition during her pregnancy, was believed to have been the first person to have the virus completely eradicated by drugs.

With some 260,000 babies born around the world each year carrying the virus, the Mississippi case had increased hopes that babies infected in the womb through their mothers could be cured upon birth with a cocktail of treatment drugs.

Unfortunately, doctors are reporting, the child in question, born in 2010, has now tested positive for the presence of the human immunodeficiency virus, news that "felt very much like a punch to the gut," says Hannah Gay, a pediatrician at the University of Mississippi Medical Center who treated the infant at birth.

DNA sequencing confirmed it is not a subsequent or later infection but is the original virus the mother passed to the child during pregnancy, she said.

"We knew it was a possibility," Gay said. "We had no way to predict if and when that might occur."

The Mississippi doctors made the original decision to treat the newborn because of the mother's HIV status even though infants are not usually treated until at least six weeks after birth when a possible HIV infection can be confirmed.

Subsequent tests showed no signs of the virus, leading to hopes that a complete cure had been achieved.

However, recent blood tests on the child, now 4, showed a significant presence of the HIV virus.

The tests showed a viral load of 16,000 copies of the virus per cubic milliliter of blood.

"You sometimes get a blip of 100 copies or 500 copies, but 16,000 is not a blip," said AIDS expert Dr. Anthony S. Fauci, head of the National Institute of Allergy and Infectious Diseases. "That is an unequivocal relapse."

"Certainly, this is a disappointing turn of events for this young child, the medical staff involved in the child's care, and the HIV/AIDS research community," Fauci said. "Scientifically, this development reminds us that we still have much more to learn about the intricacies of HIV infection and where the virus hides in the body."

In the light of the apparent failure to cure the girl, researchers say they are reexamining plans they had made for further infant drug trials to combat HIV infections.

"It's obviously disappointing, but I was not surprised," Fauci says of the news of the girl's relapse. "I've been chasing these reservoirs for the last 25 years, and I know this virus has a really uncanny way of hiding itself."

The girl is being given anti-viral treatments and is responding well, doctor say.