On Sunday night, medical researchers reported that a girl born HIV-positive had been "functionally cured." If true, and repeatable, this would be an absolutely enormous breakthrough in AIDS research, especially for sub-Saharan Africa, which has a high infant infection rate. Those are some pretty important ifs, though. Here's the story:

Two and a half years ago, a woman arrived at a rural Mississippi hospital in labor, unaware that she was infected with HIV and reportedly having received no prenatal care. The baby girl was born prematurely, and doctors tested her for HIV. But before the tests even came back positive, doctors put the baby on an aggressive treatment of three anti-retroviral drugs, starting 30 hours after birth. After about a month, the girl's viral levels had dropped to the level of being almost undetectable. After 15 months the treatment became sporadic, and after 18 months, the mother stopped bringing the girl in altogether, meaning the medication stopped. Doctors expected the level of HIV to shoot up after the treatment ended, but when the girl returned five months later, the virus was still undetectable.

The girl's pediatrician, Dr. Hannah Gay, roped in virologist Deborah Persaud at Johns Hopkins Children's Center, and Persaud and Dr. Katherine Luzuriaga, an immunologist at University of Massachusetts Medical School, performed sophisticated blood tests and discovered trace amounts of HIV remnants but none capable of replicating. That makes Gay's early, aggressive treatment a "functional cure," Persaud says — at least for this one toddler.

The girl isn't the first patient functionally cured of AIDS — but she's only the second verifiable case. And the first, a middle-aged man named Timothy Brown, became HIV-free when he received a leukemia-related bone-marrow transplant from a donor genetically resistant to the virus. "For pediatrics, this is our Timothy Brown," says Persaud. "It's proof of principle that we can cure HIV infection if we can replicate this case."

But there are unique aspects of this case that other "experiments will struggle to replicate, ethically," says James Hamblin at The Atlantic. First, "we don't have good studies on giving infants all-out anti-retroviral treatments, which can be toxic, until after we get results from blood tests that indicate they've definitely been infected." And there's the issue of taking the children off the medication after 18 months, which could cause the virus to return with a vengeance if this case was a fluke. But even though "one case is one case," the result is "great for this kid, and a day that should live in history and inspire progress."

And even if doctors can get this treatment to work in other HIV-positive newborns, "one thing is certain — this approach is not going to provide a cure for the vast majority of people with HIV," says James Gallagher at BBC News. Once someone is fully infected, the virus hides in his or her DNA, making it virtually impossible to treat. One theory is that the drugs wiped out the HIV in the girl before the virus had a chance to hide, leading some skeptics to argue that maybe she wasn't "cured."

Still, that doesn't mean that if this treatment is replicable, it won't have a huge impact. In the U.S. and other developed countries, more than 98 percent of babies born to HIV-positive moms are free of the virus, thanks to prenatal and neonatal prevention. But about 1,000 babies worldwide are born with HIV each day, primarily in sub-Saharan Africa, and more than half of those children die within a year. "Obviously there's much testing to be done, but it's something to feel hopeful about," says Laura Beck at Jezebel. "If the research successfully withstands further testing — and the drugs aren't so expensive that they're practically useless — this could be big."

The fact that this one girl appears to be free of HIV raises another possibility: With the children who have been taking drugs since birth, "it may be that we cured them and we don't realize it," says Dr. Anthony Fauci, head of the National Institutes of Health. "You don't want to recommend stopping therapy, but you do want to go back and look very carefully," he said.

And as tantalizing as this possible cure is, it's worth noting that "HIV is not the killer it used to be," says the BBC's Gallagher. "People infected with HIV should have a near normal lifespan if they have access to treatment." We need to put money into providing those life-saving drugs to the people who need them, not just toward finding a cure. But "we've started to discover things we didn't know before, and it's opening up a chink in the armor," Oxford University's John Frater tells BBC News. "A cure is something we can no longer write off as impossible."