According to a U.N. report published in August, an estimated 33 million people were living with HIV in 2007.
The report said that in virtually all regions outside sub-Saharan Africa, HIV - the virus that causes AIDS - disproportionately affected people who inject drugs, homosexual men and sex workers.
More than one million people in the United States are living with HIV/AIDS, according to the Centers for Disease Control.
United Nations Secretary-General Ban Ki-moon, speaking at a conference in Doha, paused to recognize the increase in resources that have been implemented in recent years to combat the disease, including a commitment by the United States of spending $39 billion on HIV/AIDS over the next five years.
However, the U.N. head stressed that donors and nations must uphold their commitments top the struggle against AIDS. "If not, billions of people will face devastating consequences," he said.
Marking World AIDS Day in Washington, President George W. Bush said that his presidential initiative on the deadly disease had already met its goal of treating two million people in sub-Saharan Africa.
When the administration launched the President's Emergency Plan For AIDS Relief in 2003, the goal was to support two million people with lifesaving antiretroviral treatment in five years.
"I'm pleased to announce that we have exceeded that goal early," Mr. Bush said, standing with first lady Laura Bush on the North Lawn of the White House, which was decorated with a giant red ribbon to mark the occasion.
When the program began, only 50,000 people living with HIV in all of sub-Sahara Africa were receiving antiretroviral treatment, the president said.
In addition, the U.S. supported care for more than 10 million people around the world who were affected by HIV, including more than four million orphans and vulnerable children.
Mr. Bush concluded by thanking his fellow Americas for being "so compassionate and so caring and so decent."
In videotaped remarks to the Saddleback Civil Forum on Global Health held in Washington, President-elect Barack Obama praised the Bush administration's effort to combat AIDS and pledged Monday to continue to fight the deadly disease when he takes office in January.
In the video, Obama noted advancements since the first World AIDS Day 20 years ago.
"I salute President Bush for his leadership in crafting a plan for AIDS relief in Africa and backing it up with funding dedicated to saving lives and preventing the spread of the disease," Obama said. "And my administration will continue this critical work to address the crisis around the world."
He also urged people to recommit themselves to addressing AIDS in the United States with a strategy involving prevention, treatment and a focus on at-risk communities. Obama said everyone must help address the disease because "in the end this epidemic can't be stopped by government alone, and money alone is not the answer either."
Meanwhile, at a news conference in Paris French First Lady Carla Bruni-Sarkozy said she planned to travel to meet people with AIDS and organize fundraising events in support of the Global Fund to Fight AIDS, Tuberculosis and Malaria.
Bruni-Sarkozy, who lost her brother Virginio to AIDS two years ago, accused the world of becoming complacent about the disease, which killed 300,000 children in 2007.
The model-turned-singer said her work as a goodwill ambassador would focus on women and children infected with HIV.
Earlier in the day, demonstrators had gathered on the steps of the Palais de Justice, France's highest court, to highlight the ongoing case of a French woman who is being prosecuted for infecting her husband with AIDS.
Blowing horns and chanting, they called on the government to stop penalizing those who transmit the virus, and denounced the prosecution of the woman.
Police later removed the protesters from the steps of the courthouse.
Elsewhere, public events, ceremonies and vigils marked World AIDS Day with hope and grief.
In South Africa,as the nation (whose government has for years denied the epidemic that kills 1,000 of its people every day) observed a minute of silence for AIDS victims.
In Vienna, the U.N. marked World AIDS Day with a balloon launch, as senior officials urged a group of high school students against being complacent.
Ferdinand Hascher, a student of the Vienna International School, said, "Every hour, eight people die of AIDS. And that is, for me, really shocking because you wouldn't believe that. It just happens so fast."
Meanwhile, a counter-view was proffered by some critics who say thatsuch as pneumonia and malaria.