A baby girl in the US born with HIV appears to have been cured after very early treatment with standard drug therapy, doctors say.
The Mississippi child is now two-and-a-half years old and has been off medication for about a year with no signs of infection.
More testing needs to be done to see if the treatment - given within hours of birth - would work for others.
If the girl stays healthy, it would be the world's second reported 'cure'.
Dr Deborah Persaud, a virologist at Johns Hopkins University in Baltimore, presented the findings at the Conference on Retroviruses and Opportunistic Infections in Atlanta.
The baby was born in a rural hospital where the mother had only just tested positive for HIV infection.
Because the mother had not been given any prenatal HIV treatment, doctors knew the baby was at high risk of being infected.
Researchers said the baby was then transferred to the University of Mississippi Medical Center in Jackson.
Once there, paediatric HIV specialist Dr Hannah Gay put the infant on a cocktail of three standard HIV-fighting drugs at just 30 hours old, even before laboratory tests came back confirming the infection.
"I just felt like this baby was at higher-than-normal risk and deserved our best shot," Dr Gay said.
The treatment was continued for 18 months, at which point the child disappeared from the medical system. Five months later the mother and child turned up again but had stopped the treatment in this interim.
The doctors carried out tests to see if the virus had returned and were astonished to find that it had not.
Dr Rowena Johnston, of the Foundation for Aids Research, said it appeared that the early intervention that started immediately after birth worked.
"I actually do believe this is very exciting. This certainly is the first documented case that we can truly believe from all the testing that has been done."
Many doctors in six different laboratories all applied different, very sophisticated tests trying to find HIV in this infant and no body was able to find any.
A spokeswoman for the HIV/Aids charity the Terrence Higgins Trust said: "This is interesting, but the patient will need careful ongoing follow-up for us to understand the long-term implications for her and any potential for other babies born with HIV."
US Researchers said they believe early intervention -- in this case within 30 hours of birth -- with three anti-viral drugs was key to the outcome.
A "functional cure" is when the presence of the virus is so small, life-long treatment is not necessary and standard clinical tests cannot detect the virus in the blood.
The finding was announced at the 2013 Conference on Retroviruses and Opportunistic Infections in Atlanta.
The results of the findings could possibly lead to a cure for children infected with HIV.
"We didn't have the opportunity to treat the mom during the pregnancy as we would like to be able do to prevent transmission to the baby.We are hoping that future studies will show that very early institution of effective therapy will result in this same outcome consistently." said Dr Hannah Gay.
"This is a proof of concept that HIV can be potentially curable in infants," she said.
In 2007, Timothy Ray Brown became the first person in the world believed to have recovered from HIV.
His infection was eradicated through an elaborate treatment for leukaemia that involved the destruction of his immune system and a stem cell transplant from a donor with a rare genetic mutation that resists HIV infection.
In contrast, the case of the Mississippi baby involved a cocktail of widely available drugs already used to treat HIV infection in infants.
It suggests the treatment wiped out HIV before it could form hideouts in the body.
These so-called reservoirs of dormant cells usually rapidly re-infect anyone who stops medication, said Dr Persaud.
Dr Katherine Luzuriaga, an immunologist at the University of Massachusetts who worked closely with Gay, called the developments fascinating, including the fact that the toddler was found to have no virus in her blood even after her mother stopped giving her treatment for eight to 10 months.
"We really can quite confidently conclude at this point that the child does very much appear to be cured"
Dr Rowena Johnston
Foundation for Aids Research