HIV is present mainly in blood and semen, but is only found in low levels in saliva, making kissing a low risk for transmission of the virus.

Scientists have long questioned why.

The discovery that the enzyme lysozyme stops the spread of HIV may be the answer.

The enzyme has long been known to be present in tears and can be found in large quantities in the urine of pregnant women.

Urine answer

Researchers from New York University School of Medicine and the US National Institutes of Health have been searching for five years for the answer to why pregnant women's urine is resistant to HIV.

At first they thought it was due to a hormone produced by the placenta.

This was found to stop the spread of HIV in the laboratory and to shrink tumours caused by Kaposi's sarcoma, a skin cancer linked to Aids.

It would also have explained why HIV transmission is rare in the first three months of pregnancy when the hormone is present in high levels.

But highly concentrated forms of the hormone were shown to have no particular effect on HIV-infected cells.

The scientists then investigated other substances present in urine, including lysozyme.

They tested purified forms of the enzyme for two years and found that it and other proteins, called ribonucleases, combined to fight HIV.

Fleming

Lysozyme was first identified as an anti-bacterial agent in 1922 by Alexander Fleming, who discovered penicillin.

He noted that it broke down the cell walls of bacteria, causing them to burst and die.

Ribonucleases are though to break apart the genetic material of viruses like HIV.

The scientists, led by Dr Sylvia Lee-Huang, do not know how the proteins work on HIV, but they suspect that lysozyme breaks down the outer membrane of the virus so that it cannot invade the immune system.

Ribonculeases may then stop the virus from reproducing.

The finding, which is published in The Proceedings of the National Academy of Sciences, may lead to the development of a new class of anti-Aids drugs.

Dr Lee-Huang is very hopeful about the drugs. "These proteins are very promising anti-Aids agents and likely to be well tolerated by the body, causing few side effects because they occur naturally," she said.

The next step in the research is to start developing treatments using the proteins.

Effective treatments

Jeffrey Williams, a health promotion officer and treatment specialist at the Terrence Higgins Trust, said the findings were "very, very interesting".

But he added that it remained to be seen if the proteins could be turned into effective treatments.

"You cannot just inject a foreign protein into the body. It can produce a violent reaction.

"It is all very well showing that this enzyme is very active against HIV, but another trying to use it as medication."

However, he said the findings suggested that the body was capable of providing its own natural defences against disease.

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The original of this article can be found at http://news.bbc.co.uk/hi/english/health/background_briefings/aids/newsid_297000/297307.stm.

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