Kaposi's sarcoma, one of the commonest cancers in young men in the West, is almost certainly caused by a human herpes virus, scientists from the Institute of Cancer Research and University College London have shown.
They believe that the KS virus is transmitted by sexual activity and lies dormant until it is triggered into replicating if the carrier's immune system becomes suppressed - most commonly when they develop Aids.
The finding helps to solve the mystery of Kaposi's sarcoma, a tumour of the small blood vessels which often appears in the skin. It is prevalent in gay men with Aids but not in haemophiliacs with Aids, and is sometimes found in people who are not HIV positive.
The scientists, led by Thomas Schulz at ICR, say they can now predict, through a blood test, which HIV-positive gay men will develop Kaposi's sarcoma.
Professor Schulz, who has just become professor of genito-urinary medicine at Liverpool University, said that the nature of the link between Aids and KS has been disputed: "People have always argued that there must be another agent out there." Since the 1970s many candidates have been put forward, only to be discounted.
But last year, scientists at Columbia University in New York took biopsies of KS sufferers and fished out small pieces of a virus similar to other viruses known to cause tumours.
Scientists realised that if they could show that these viral pieces could be found in every KS sufferer, they would have good evidence that they had found its cause.
Professor Shulz said that this link had now been demonstrated: "The most important point of our research is that if you are an HIV-positive gay man without visible Kaposi's sarcoma, and we can detect that virus in your blood, then you're much more likely to progress to KS than others.
"This is good evidence for a causal link."
Robin Weiss, director of research at the institute, said: "Being able to predict who will develop KS may lead to early treatment and the discovery of this virus may one day enable a vaccine to be developed."