The results, described as "absolutely fascinating" by a leading British researcher into Aids, build on work published this year by a Glaswegian scientist working at one of the world's top Aids research centres, in New York. In March, Dr Bill Paxton, at the Aaron Diamond Research Center reported on a group of 15 people who, despite having been exposed to the virus many times, remained uninfected and healthy.
Now, further genetic tests on two of the 15 have found they have a flaw in a matched pair of genes, leading to cells in their immune system lacking a particular surface protein - which HIV normally uses to infect the cells. The matched flaw is reckoned to occur in about one in every 100 Western Europeans.
Its apparent effect is to make it impossible for HIV to infect them, said Dr Nathaniel Landau, who also works at the Aaron Diamond Center. His research on the two men - Steve Crohn and Eric Fuchs - is published in today's edition of the science journal, Cell.
But the immunity only occurs if both copies of the gene, known as CCR- 5, have the defect - known as being "homo- zygous". If only one of the pair of genes has the flaw, the cells have the surface protein and HIV can infect them, but the progression is much slower.
The result is that in homo- zygous people the virus, which needs to infect cells in order to multiply and become infectious to others, is effectively neutralised. People who were immune in this way could not transmit the disease.
"We have to do more studies in the general population, because so far we have only tested a few hundred people," said Sidney Ho, general manager of the Aaron Diamond Center.
Studies by a Belgian team led by Marc Parmentier in Brussels found that the gene defect was not present in 124 Africans and 248 Japanese, suggesting it is only found in those of European origin. An estimated one in five Europeans has one copy of the flawed gene.
"It's fascinating, because it provides us with a mechanism for the truth we have observed - some people have multiple exposures to HIV but remain uninfected," said Dr Brian Gazzard, clinical research director of the HIV department at Chelsea and Westminster Hospital.
The cause of the genetic flaw is unknown, but it may be an accidental mutation. It consists of a "deletion" in a gene which instructs the white blood cells, known as CD4 cells, to make a protein known as CKR5.