If the therapy is going to work, they have to be religious about taking every dose

Dr Hermione Lyall, St Mary's Hospital

The un-named three-year old is now being examined in hospital so that doctors can determine how the HIV has affected her.

Keith Alcorn, of the National Aids Manual, which advises professionals and patients on HIV treatment advice, said: "The treatment is not so different for a child of three as it would be for an adult - though for small babies it is different.

"Doctors will be looking at the levels of the virus in the blood and the state of her immune system."

Survival rates

"If the immune system damage is fairly advanced, she is vulnerable to bacterial infections or Cytomegalovirus, (a common virus which can cause problems in people with weakened systems), which could cause eye damage."

The child is now having hospital tests

He said there had also been reports that Australian doctors had said there was some neurological damage - and tests would be carried out to see if that were the case.

Movement could be affected, or there could be learning difficulties.

Mr Alcorn added: "If any of these symptoms were present, then - in the opinion of most doctors in this country, that would be an indication of the need for treatment."

Dr Hermione Lyall of the Family HIV Centre at St Mary's Hospital, London, told BBC News Online: "Twenty per cent of children will actually reach the serious stage of the disease in the first year of life, developing severe infections - Aids."

But she said if they received urgent treatment, they tended to do well.

She added: "In Europe, 50% of children will still be alive and relatively well by the time they're 10-years-old, even before they've had combination therapy."

The girl's mother, who died last year, had ignored conventional medical advice to take drugs during pregnancy and not to breastfeed, so the risk of transmission would be removed.

Mother-to-child transmission can be largely prevented by anti-retroviral treatment during pregnancy.

Treatment regimes

Once it has been decided that a child needs to take combination therapy treatment, doctors and the family must realise what a commitment that is.

Adults usually take a combination of three drugs. Sometimes, children need to take four, because the levels of HIV virus in the blood can be extremely high.

There are three classes of anti-retroviral drugs which can be given.

Nucleoside analogues and non-nucleoside reverse transcriptase inhibitors work in different ways to prevent the HIV virus integrating with the patient's DNA.

Protease inhibitors work by preventing the virus from assembling itself.

What combination a child receives depends on their age and how advanced their illness is.

If a baby or child is given four drugs, the treatment is usually made up of three from one class and one from another.

The drugs, which for children often come in liquid form, must be taken twice a day.

The father of the HIV-positive girl at the centre of the current case is against the treatment.

But Mr Alcorn said: "The role of the carer in administering the medication, and ensuring that the schedule is kept up with, is absolutely critical."

Dr Lyall warned the family has to stick to the treatment: "If the therapy is going to work, they have to be religious about taking every dose."