By Julie Steenhuysen

CHICAGO (Reuters) - Instead of infiltrating breaks in the skin, HIV appears to attack normal, healthy genital tissue in women, U.S. researchers said on Tuesday in a study that offers new insight into how the AIDS virus spreads.

They said researchers had assumed the human immunodeficiency virus, or HIV, sought out breaks in the skin, such as a herpes sore, in order to gain access to immune system cells deeper in the tissue.

Some had even thought the normal lining of the vaginal tract offered a barrier to invasion by the virus during sexual intercourse.

"Normal skin is vulnerable," Thomas Hope of Northwestern University's Feinberg School of Medicine said in a telephone interview.

"It was previously thought there had to be a break in it somehow," said Hope, who is presenting his findings at a meeting of the American Society for Cell Biology in San Francisco.

He said until now, scientists had little understanding of the details of how HIV is transmitted sexually in women.

Hope and colleagues at Northwestern in Chicago and Tulane University in New Orleans developed a new method for seeing the virus at work. They studied newly removed vaginal tissue taken from hysterectomy surgeries, and introduced the virus which carried fluorescent, light-activated tracers.

They watched under a microscope as the virus penetrated the outer lining of the female genital tract, called the squamous epithelium. They also observed the same process in nonhuman primates.