This is what motivates Sharon Hillier.

As she was leaving Durban, South Africa, recently, she encountered a young woman in a store who noticed that Dr. Hillier was carrying a bag with the word "microbicides" on it.

The woman knew what microbicides were -- a medication that can kill germs, and in particular, can target HIV, the virus that causes AIDS.

"I wish I had a microbicide," she told Dr. Hillier, a University of Pittsburgh professor who runs one of the world's largest research trials into using the substances to prevent HIV infections.

"I asked her why," Dr. Hillier recounted. "She said, 'Well, my sister's just gotten sick. Her husband died last year, and we think he had AIDS, and now my sister's sick and she has four children, and I can't get married because I know I'm going to have to take two of her children and my mother will take two."

These are the kinds of issues that face thousands of women around the globe, Dr. Hillier says. Unless they can find an effective way to keep from getting AIDS, they have to forgo their own dreams so they can be available to take care of others.

"Young women are desperate for ways to have a life we all take for granted," said Dr. Hillier, whose Microbicide Trials Network is based at the Magee-Womens Research Institute in Oakland.

That is also what spurs her on in her search for a substance that will reliably prevent HIV infections and can be put into a vaginal gel, a pill, an injection or a contraceptive ring.

As much as any HIV prevention trial being funded by the federal National Institutes of Health, the microbicide effort is aimed at women, addressing what she calls "the feminization of the HIV epidemic" that has been under way for more than a decade.

Because three-quarters of HIV infections in the United States still occur among men, people here often lose sight of the fact that the global picture is very different.

Around the world, the United Nations estimates, half of those living with AIDS are women. And in sub-Saharan Africa, which accounts for most new cases, there are 13 million women with HIV, compared with 9 million men.

One of the persistent myths about the global epidemic, Dr. Hillier said, is that most women with HIV are prostitutes or promiscuous.

But the typical victim in Africa or India, she said, is a woman faithfully married to one man -- a man who has had sex with other women while they were married, or who was already infected when they were betrothed. For those women, she said, "the old prevention mantra of abstinence, being faithful and using condoms just doesn't work."

That's why she hopes to develop a reliable microbicide that can give women control over their own destinies.

It's been a difficult challenge so far.

The work began several years ago with the use of a well-known spermicide called nonoxynol-9. If it could kill sperm, then it certainly could kill the HIV carried by the sperm, the reasoning went.

Instead, a 2001 study showed, nonoxynol not only didn't prevent HIV infections, but actually increased them in some women, apparently by inflaming the vaginal linings.

Since then, Dr. Hillier said, her project has tried other substances. Three have failed -- two other spermicides and one substance designed to block the entry of viruses into cells.

It's a different story for another substance that the network is now investigating, an antiviral agent known as tenofovir. It's already known to work against HIV and is part of the three-drug cocktail that many patients take to hold the disease in check.

The network is about to launch a prevention trial to see whether the drug is more effective as a pill or a vaginal gel, she said, and hopes to get solid results in three to four years.

A recently finished trial in the United States and India with 200 women showed that the gel was safe and that none of the women got an HIV infection.

Just as important, the women found the gel easy to use each day, and "they found it never made sex worse, and sometimes it made sex better."

If a daily pill or application of gel is feasible, Dr. Hillier said, it will take away one of the main problems with any prevention method that has to be used right before sex.

"I think there are a substantial number of people at high risk of HIV who aren't going to use something right before they have sex because it interrupts the moment and because it says [to a partner] I don't really trust you, whereas if you use something every day it's not that I don't trust you, but I just don't want to get burned."

The main reason fewer women have HIV in America is that the overall prevalence is so much lower. The HIV prevalence among adults in North America is less than 1 percent. In sub-Saharan Africa, it is 5 percent -- eight times higher.

Sexual practices in those two parts of the world are not that much different, though, Dr. Hillier said, even though many Westerners mistakenly and condescendingly think they are.

Sometimes, when she is reading her HIV documents on a plane, another passenger will take notice, "and inevitably they will say, 'Wouldn't it be good if you just told those people how you get HIV because then they would know better.'

"And I always take a deep breath and say, OK, dude, you just bought yourself a 45-minute lecture on some reality testing, because then I turn to them and say, 'Did you use condoms the last time you had sex?' and they'll say no, and I'll say, 'Why not?' and they'll say, 'Because my partner's not infected,' and I'll say, 'Do you know that?' "

The attitudes men have about their sexual rights in other cultures do play a part in the epidemic, she said, but there are many women in the United States and other industrialized nations who face the same pressures from their partners.

Dr. Hillier has grown up with the HIV epidemic, professionally speaking.

While working on her Ph.D. at Washington State University, she studied the bubonic plague, which still can be found in various parts of the world. As she interviewed for postdoctoral research jobs in the early 1980s, though, there was a deadly new disease that people were just starting to talk about, the infection that turned out to be AIDS.

She went on to specialize in sexually transmitted diseases and how some of them trigger premature births, and that led her eventually into HIV research.

Dr. Hillier feels her group's work is all the more crucial because it now appears it will be a long time before there is an effective HIV vaccine.

"We used to say we need microbicides until we have a vaccine, but my view is that we really need to invest a lot of resources into coming up with prevention methods, because in 25 more years we're still not going to have an effective vaccine."

Despite that, she remains hopeful that HIV prevention will one day succeed.

"I do have tremendous hope; I believe we'll have a victory. I think the American people should feel really proud that this is something good that we're doing and it's exciting that Pittsburgh is the place we're doing it."