CHICAGO (Reuters) - Instead of infiltrating breaks in the skin, HIV appears to attack normal, healthy genital tissue, 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 beaks 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,” said 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 the American Society for Cell Biology meeting 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.

Then they watched under a microscope as the virus penetrated the outer lining of the female genital tract, called the squamous epithelium. They also observed this same process in non-human primates.

In both cases, they found HIV was able to quickly move past the genital skin barrier to reach immune cells, which the virus targets.

Hope said the study suggests the virus takes aim at places in the skin that had recently shed skin cells, in much the same way that skin on the body flakes off.

The finding casts doubt on the prior theory of the virus requiring a break in the skin or gained access through a single layer of skin cells that line the cervical canal.

And it might explain why some prevention efforts have failed. Hope said one clinical trial in Africa in which women used a diaphragm to block the cervix had no effect at reducing transmission of the virus. Nor have studies of drugs designed to prevent lesions in genital herpes proven effective.

Hope said the findings emphasize the need for treatments such as a vaccine to prevent infection.

And it makes clear the need for the use of condoms, which are highly effective at preventing infection.

“People need to remember that they are vulnerable,” Hope said. “The sad part is if people just used a condom, we wouldn’t have this problem,” he said.

In the United States, HIV is mostly passed among men who have sex with men. Females account for 26 percent of all new HIV cases in the United States, according to the U.S. Centers for Disease Control and Prevention.

Globally, HIV is more commonly spread by heterosexual sex. The virus has infected 33 million people globally and has killed 25 million.