The AIDS virus is confirmed to have returned to a baby born in Mississippi previously believed to be cured of the disease.

According to NBC News, the now-four-year-old girl was showing signs of the AIDS virus attacking her immune system. For most of her young life, the girl was constantly tested for AIDS after a treatment of three drugs normally used for the advanced stages of the virus.

"It felt very much like a punch to the gut," Dr. Hannah Gay, a University of Mississippi researcher who had been treating the baby girl, told NBC News.

The little girl was born prematurely to a woman who only learned she had AIDS during labor. Doctors confirmed within 30 hours of the baby's birth that she too had the virus. In a decision unique to the baby with AIDS, doctors used the treatment cocktail and it lowered her levels to "untraceable."

"The baby has now rebounded with clearly detectable HIV viremia," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), told NBC News. "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."

Doctors in Long Beach, Calif. used the same method in 2013 by treating a newborn baby whose mother they knew was not taking her HIV medication. The baby has not showed signs of AIDS again since birth.

Deborah Persaud, professor of infectious diseases at the John Hopkins Children's Center in Baltimore, told USA Today the Mississippi baby may still be a positive signal in the fight against AIDS. Researchers investigating the case said it might still be important for AIDS patients to never assume they are cured and should maintain their medication schedule.

"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," Persaud said. "Typically, when treatment is stopped, HIV levels rebound within weeks, not years."