Doctors in the US have made medical history by effectively curing a child born with HIV, the first time such a case has been documented.

The infant, who is now two and a half, needs no medication for HIV, has a normal life expectancy and is highly unlikely to be infectious to others, doctors believe.

Though medical staff and scientists are unclear why the treatment was effective, the surprise success has raised hopes that the therapy might ultimately help doctors eradicate the virus among newborns.

Doctors did not release the name or sex of the child to protect the patient's identity, but said the infant was born, and lived, in Mississippi state. Details of the case were unveiled on Sunday at the Conference on Retroviruses and Opportunistic Infections in Atlanta.

Dr Hannah Gay, who cared for the child at the University of Mississippi medical centre, told the Guardian the case amounted to the first "functional cure" of an HIV-infected child. A patient is functionally cured of HIV when standard tests are negative for the virus, but it is likely that a tiny amount remains in their body.

"Now, after at least one year of taking no medicine, this child's blood remains free of virus even on the most sensitive tests available," Gay said.

"We expect that this baby has great chances for a long, healthy life. We are certainly hoping that this approach could lead to the same outcome in many other high-risk babies," she added.

The number of babies born with HIV in developed countries has fallen dramatically with the advent of better drugs and prevention strategies. Typically, women with HIV are given antiretroviral drugs during pregnancy to minimise the amount of virus in their blood. Their newborns go on courses of drugs too, to reduce their risk of infection further. The strategy can stop around 98% of HIV transmission from mother to child.

In the UK and Ireland, around 1,200 children are living with HIV they picked up in the womb, during birth, or while being breastfed. If an infected mother's placenta is healthy, the virus tends not to cross into the child earlier in pregnancy, but can in labour and delivery.

The problem is far more serious in developing countries. In sub-Saharan Africa, around 387,500 children aged 14 and under were receiving antiretroviral therapy in 2010. Many were born with the infection. Nearly 2 million more children of the same age in the region are in need of the drugs.

In the latest case, the mother was unaware she had HIV until after a standard test came back positive while she was in labour. "She was too near delivery to give even the dose of medicine that we routinely use in labour. So the baby's risk of infection was significantly higher than we usually see," said Gay.

Doctors began treating the baby 30 hours after birth. Unusually, they put the child on a course of three antiretroviral drugs, given as liquids through a syringe. The traditional treatment to try to prevent transmission after birth is a course of a single antiretroviral drug. The doctor opted for the more aggressive treatment because the mother had not received any during her pregnancy.

Several days later, blood drawn from the baby before treatment started showed the child was infected, probably shortly before birth. The doctors continued with the drugs and expected the child to take them for life.

However, within a month of starting therapy, the level of HIV in the baby's blood had fallen so low that routine lab tests failed to detect it.

The mother and baby continued regular clinic visits to the clinic for the next year, but then began to miss appointments, and eventually stopped attending all together. The child had no medication from the age of 18 months, and did not see doctors again until it was nearly two years old.

"We did not see this child at all for a period of about five months," Gay told the Guardian. "When they did return to care aged 23 months, I fully expected that the baby would have a high viral load."

When the mother and child arrived back at the clinic, Gay ordered several HIV tests, and expected the virus to have returned to high levels. But she was stunned by the results. "All of the tests came back negative, very much to my surprise," she said.

The case was so extraordinary, Dr Gay called a colleague, Katherine Luzuriaga, an immunologist at Massachusetts Medical School, who with another scientist, Deborah Persaud at Johns Hopkins Children's Centre in Baltimore, had far more sensitive blood tests to hand. They checked the baby's blood and found traces of HIV, but no viruses that were capable of multiplying.

The team believe the child was cured because the treatment was so potent and given swiftly after birth. The drugs stopped the virus from replicating in short-lived, active immune cells, but another effect was crucial. The drugs also blocked the infection of other, long-lived white blood cells, called CD4, which can harbour HIV for years. These CD4 cells behave like hideouts, and can replace HIV that is lost when active immune cells die.

The treatment would not work in older children or adults because the virus will have already infected their CD4 cells.

"Prompt antiviral therapy in newborns that begins within days of exposure may help infants clear the virus and achieve long-term remission without lifelong treatment by preventing such viral hideouts from forming in the first place," said Dr Persaud. "Our next step is to find out if this is a highly unusual response to very early antiretroviral therapy or something we can actually replicate in other high-risk newborns."

Children infected with HIV are given antiretroviral drugs with the intent to treat them for life, and Gay warned that anyone who takes the drugs must remain on them.

"It is far too early for anyone to try stopping effective therapy just to see if the virus comes back," she said.

Until scientists better understand how they cured the child, Gay emphasised that prevention is the most reliable way to stop babies contracting the virus from infected mothers. "Prevention really is the best cure, and we already have proven strategies that can prevent 98% of newborn infections by identifying and treating HIV-positive women," she said.

Genevieve Edwards, a spokesperson for the Terrence Higgins Trust HIV/Aids charity, said: "This is an interesting case, but I don't think it has implications for the antenatal screening programme in the UK, because it already takes steps to ensure that 98% to 99% of babies born to HIV-positive mothers are born without HIV."