Men can transmit the Aids virus far more easily to women than previously believed, say researchers at Northwestern University, who found that the virus can travel through a healthy female's genital skin and reach its immune cell targets in four hours.

Scientists had long believed that the normal lining of the female vaginal tract was an effective barrier to invasion of the HIV virus during sexual intercourse. They thought the large HIV virus couldn't penetrate the tissue.

But new research from Northwestern University's Feinberg School of Medicine has shown for the first time that the HIV virus does indeed penetrate a woman's normal, healthy genital tissue to a depth were it can gain access to its immune cell targets.

"This is an unexpected and important result," said Thomas Hope, principle investigator and professor of cell and molecular biology at the Feinberg School.

"We have a new understanding of how HIV can invade the female vaginal tract. Until now, science has really had no idea about the details of how sexual transmission of HIV actually works. The mechanism was all very murky," the expert added.

Hope, his Northwestern colleagues, and collaborators at Tulane University discovered that interior vaginal skin is vulnerable to HIV invasion at the level where it naturally sheds and replaces skin cells, a point where the cells are not as tightly bound together.

The study has been presented at the American Society for Cell Biology 48th annual meeting in San Francisco.