The child continued to stay off treatment and showed no sign of the virus for more than two years.
"Typically, when treatment is stopped, HIV levels rebound within weeks, not years," said Deborah Persaud, professor of infectious diseases at the John Hopkins Children's Center in Baltimore.
Persaud described the child's response as "unprecedented."
Now age four, she was tested during a routine clinical care visit earlier this month, and was found to have detectable HIV levels in her blood.
She also had a decreased T-cell count and the presence of HIV antibodies, signaling that her body was fighting the infection and that HIV was actively replicating again in her body.
The girl, whose identity has not been released, is now being treated once again with anti-retroviral medication and is doing well, Fauci said.
"The case of the Mississippi child indicates that early anti-retroviral treatment in this HIV-infected infant did not completely eliminate the reservoir of HIV-infected cells that was established upon infection but may have considerably limited its development and averted the need for anti-retroviral medication over a considerable period," said Fauci.