I have met many remarkable people in my life: presidents, kings, diplomats. One of the most memorable of these encounters - and certainly most moving - came a bit more than a year ago, when I met a group of HIV-positive staff members at the United Nations.

For me, it was a moment of epiphany. I was struck by their courage and, more, by their directness in talking about their lives. Suddenly, I saw the human face of HIV. It made me wonder: What could I do differently, how could I help?

It was impossible not to be reminded of the discrimination those with HIV often face in many parts of the world, including Asia. Against this backdrop, I was so proud of my U.N. colleagues - for standing up and speaking out, for challenging stigma and discrimination and for helping make the U.N. a model of how the workplace should respond to AIDS.

This week, more than 20,000 activists, academics and policy-makers gathered in Mexico City for the 17th International AIDS Conference, heralding what many called a "new era" in fighting the disease. For the first time fewer people are being infected by HIV and fewer are dying. A recent UNAIDS 2008 Report shows encouraging progress in preventing HIV in a number of the most vulnerable countries, thanks to changes in sexual behavior (particularly among young people) and better access to anti-retroviral drugs.

These gains would not have been possible without strong support from the international community. In Hokkaido last month, G8 nations renewed their pledge to work toward the goal of universal access to HIV treatment by 2010. More recently, President Bush signed legislation committing $48 billion to the fight against AIDS, tuberculosis and malaria over the next five years. And yet, we must remember: One of the biggest hurdles for our global response to AIDS is psychological.

That is the stigma factor. To greater or lesser degrees, almost everywhere in the world, discrimination remains a fact of daily life for people living with HIV. One-third of all countries have virtually no laws protecting their rights. Almost all permit at least some form of discrimination - against women and children who contract the disease, against gay men, against communities at risk.

Stigma remains the single most important barrier to public action. It is the main reason too many people are afraid to see a doctor to determine whether they have the disease, or to seek treatment if so. It helps make AIDS the silent killer, because people fear the social disgrace of speaking about it, or taking easily available precautions. Stigma is a chief reason the AIDS epidemic continues to devastate societies around the world.

We can fight stigma. Enlightened laws and policies are key. But it begins with openness, the courage to speak out. Fortunately, more and more people are finding their voices, like my UN colleagues.

Visiting the AIDS conference, I was greeted with a kiss by a young activist from Honduras, just 12 years old. Infected since birth, Keren Gonzalez not only lives with HIV but thrives. She edits a magazine for children affected by AIDS (funded by UNICEF) and participates in workshops on AIDS awareness and sensitivity across Latin America. She knows only too well the pain of stigma. When she entered nursery school, teachers told her to sit in the corner and not touch other children's' books or toys. But with understanding came acceptance. "I'm the most popular girl in my class," she says with pride. Her greatest fear isn't her condition but how she will be received when she moves on to secondary school.

At the Global Village, a center for community activism at the conference, there were others - dancers, civil society leaders, even hairdressers - living with HIV, richly and happily and openly. Among them was a woman from Malawi, Maroc Daphane Jwonde, who learned she had the disease in 1999 after her husband grew sick. Fighting discrimination ever since - one co-worker asked her not to use the dishes in their shared kitchen - she asked me to use her story to "make change in the world."

Such people are at the heart of the global campaign against AIDS. It is impossible not to admire their courage and commitment. Yet their efforts, alone, are not enough.

In Mexico City, I called on world leaders to join them, to speak out against discrimination and to guarantee the rights of people living with HIV. Schools should teach respect and understanding. Religious leaders should preach tolerance. The media should condemn prejudice and use its influence to advance social change, from securing legal protections to ensuring access to health care.

Above all, we must recognize that those who bear the stigma of HIV should not be those who live with the disease. It is those who allow it.