It had been believed that only damaged skin inside the vagina could provide a route to infection. However, researchers say HIV can get past this intact barrier within hours. British HIV charities said it reinforced the need for women to avoid unprotected sex unless their partner's health is known. The lining of the vagina - the squamous epithelium - had been believed by many to be capable of keeping HIV at bay. While transmission of the virus from men to women through unprotected sex is commonplace in many parts of the world, it was thought that HIV was most likely travelling through cuts or sores in the vaginal tract, or penetrating a much thinner layer of skin further up the reproductive tract. The scientists from Northwestern University in Chicago found that, far from this being the truth, HIV could move quickly between the skin cells themselves. The weak point, they said, occurs when skin cells are about to be shed, as the cells are no longer as tightly bound together. Using HIV "tagged" with a marker which gives off light, they observed that, within four hours, the virus had reached a fraction of a millimeter below the surface. At this depth, according to the researchers, it could encounter the immune cells it needed to invade to establish itself in the body. Professor Thomas Hope, leading the research, said: "This is an important and unexpected result - we have a new understanding of how HIV can invade the female vaginal tract."
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