A Mississippi toddler born with HIV and deemed functionally "cured" by scientists now shows signs of the virus in her blood stream, doctors said. 

Dr. Hannah Gay, who first treated the child, described the news as "a punch to the gut" during a telephone conference held by the National Institutes of Health Thursday.

In March 2013, scientists declared that the girl, now 4, no longer contained any traces of the disease despite her mother disappearing with the child and halting her medication when she was 15-18 months old.

The landmark case was presented at the 2013 Conference on Retroviruses and Opportunistic Infections in Atlanta as a potentially groundbreaking development, leading many to wonder if researchers were on their way to discovering a cure. 

Many doctors, including Dr. Rowena Johnston, the director of The Foundation for AIDS Research, pointed to an aggressive treatment of antiretroviral drugs just hours after the baby's birth as the leading factor in the ultimate eradication of the virus. 

"If one had to make an educational guess, the difference was receiving the treatment dose very soon after birth, earlier than standard of care in the U.S.," Johnston told ABC news, speaking about the three drug treatment.

The mother learned that she carried the virus right before delivery and therefore received no prenatal care, which significantly reduces the risk of transmission. Those who receive adequate treatment pass on the virus in less than 2 percent of cases.

Despite the setback, the technique of administering high doses of antiretrovirals immediately after birth is still being used to combat the spread of HIV in infants. Last March, a baby born in Long Beach, California who received a similar treatment also appeared to have been cured when no traces of the virus remained in her body.