TUESDAY, Dec. 16 (HealthDay News) -- A new route of male-to-female transmission of HIV -- in which the virus can travel through healthy genital skin to reach immune cells in just four hours -- has been identified by U.S. researchers.

It's long been believed that the normal lining of the vaginal tract was an effective barrier to HIV during sexual intercourse, because the large HIV virus couldn't penetrate the tissue. But the Northwestern University Feinberg School of Medicine researchers found that HIV can penetrate normal, healthy genital tissue to a depth where it can get to immune cells and infect them.

The researchers labeled HIV viruses with photo-activated fluorescent tags and were able to track the viruses as they penetrated the outermost lining of the female genital tract (the squamous epithelium) in female human tissue obtained through hysterectomy and in animal models.

The findings were expected to be presented Dec. 16 at the American Society for Cell Biology annual meeting, in San Francisco.

"This is an unexpected and important result. We have a new understanding of how HIV can invade the female genital tract," principal investigator Thomas Hope, a professor of cell and molecular biology, said in a university news release.

"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," Hope said.

These findings, if confirmed in future studies, could help in the development of new microbicides and vaccines to protect women against HIV.

"We urgently need new prevention strategies or therapeutics to block the entry of HIV through a woman's genital skin," said Hope. While condoms are 100 percent effective in blocking HIV, "people don't always use them for cultural and other reasons."

Women account for 26 percent of all new HIV cases in the United States, according to the U.S. Centers for Disease Control and Prevention. Based on its most recent analysis of 2005 data, the CDC estimated that there were 56,300 new HIV infections that year, and 31 percent were due to high-risk heterosexual contact. More than half of the new cases of HIV infection worldwide are in women.