A baby who doctors had hoped may have been cured of HIV infection is infected again, doctors reported on Thursday — a disappointing blow to hopes that it might be possible to stop infection in its tracks.

The child, now 4, had been regularly tested for the AIDS virus and now the virus has not only returned, but showed signs of damaging her immune system, researchers said.

"It felt very much like a punch to the gut," said Dr. Hannah Gay of the University of Mississippi, who has been treating the child.

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“The baby has now rebounded with clearly detectable HIV viremia,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID).

“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.”

The Mississippi baby was born a little premature to a mother discovered to have HIV only when she was in labor. The mom had not ever been treated for HIV. Within 30 hours of birth the baby was re-tested and had clear evidence of HIV infection.

Unusually, she then got a cocktail of three drugs at a dose normally reserved for more advanced cases. It worked really well — pushing her virus down to what’s called undetectable levels.

"It felt very much like a punch to the gut."

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The baby and her mom, who doctors have never named, got regular care and treatment from Gay at University of Mississippi Medical Center until she was 15 months old. Then, like so many children, she disappeared off the doctors’ radar screens. The mother brought her back briefly at 18 months but disappeared again but she missed at least eight months' worth of drugs. When Gay caught up to her again, the baby was still well, despite having received no treatment.

Now, at age 4, she has shown evidence of infection for the first time since she was born.

In 2013, a second child was born in Long Beach, Calif. to a woman who doctors knew was not taking her HIV medication. The infant was treated immediately and the virus is now barely detectable.

The U.S. government is looking for more babies to test the treatment, but Fauci said NIAID may have to adjust the new study. "We are going to take a very careful look at that study," Fauci told reporters.

They'll be studying the child's blood and her immune system.

“Typically, when treatment is stopped, HIV levels rebound within weeks, not years.”

It's not a total loss, the researchers said. The child was able to get through the toddler years without having to take the cocktail of drugs usually used to control the virus.

“The fact that this child was able to remain off antiretroviral treatment for two years and maintain quiescent virus for that length of time is unprecedented,” said Dr. Deborah Persaud of John Hopkins Children’s Center in Baltimore and one of the two pediatric HIV experts involved in the case. “Typically, when treatment is stopped, HIV levels rebound within weeks, not years.”

The early treatment may have at least slowed the virus down, the researchers speculated.

“How can someone be off therapy for 27 months…and yet the virus remains suppressed?” Fauci asked. The baby may have had a special immune response, he 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,” Fauci added. “The NIH remains committed to moving forward with research on a cure for HIV infection.”

There was evidence the virus had started damaging the child’s immune system as it came back. She had fewer than the normal number of CD4 T-cells – the infection-fighting cells that HIV targets and kills. “We know that the CD4 T-cells dropped a bit with the re-emergence of virus but they have started to come up very nicely with the initiation of treatment,” Gay said.

“For now, the child needs antiretroviral treatment,” Persaud said. But she hopes research will find a better way to control the virus. “These children can live to their 30s and 40s,” she pointed out. The hope is to extend that as doctors learn more.

The human immunodeficiency virus that causes AIDS has only been common since the early 1980s. There’s no cure and no vaccine yet.

One of the many ways the AIDS virus is transmitted is during birth. An infected mother can infect her baby. But giving both mother and baby a few doses of two HIV drugs — AZT and nevirapine — can reduce this transmission by 99 percent.

In 2010, 162 babies were born with HIV in the 46 states where monitoring is done, the Centers for Disease Control and Prevention says. CDC says about 1.2 million people in the U.S.are infected with HIV and about 50,000 new infections are diagnosed each year.

Some experts had questioned whether the child had truly been born infected. “The one thing we did learn throughout all of this was there was some doubt the baby was infected. The baby clearly was infected,” Fauci said.

The case still suggests that a cure is possible, advocacy groups said. "The Elizabeth Glaser Pediatric AIDS Foundation expresses disappointment in this setback but remains hopeful that the scientific breakthrough that allowed the child’s HIV levels to remain undetectable for more than two years will continue to help researchers understand how to control HIV and ultimately develop a cure,"the group said in a statement.