Perhaps the most telling aspect of the Third International Conference on AIDS is that so many people are here.

More than 6,000 participants from all over the world -- researchers, social workers, nurses, doctors, epidemiologists, economists, government officials -- descended on the Washington Hilton for what has become the largest international scientific gathering ever devoted to one disease.

The organizing committee originally planned for about 3,500 participants. After all, there were 2,200 people at the first AIDS conference in Atlanta two years ago and 2,800 last year at the second in Paris. But the politics and science of AIDS have evolved rapidly in the past year, and as the third AIDS conference opened yesterday, registration topped 6,350, including 780 members of the press.

"It's an indication of the seriousness of the disease," said Dr. George J. Galasso, associate director of the National Institutes of Health and general chairman of the conference.

"This conference will be regarded as a watershed event," said Surgeon General C. Everett Koop.

From the start, Koop was the hero of the conference and received a standing ovation from the crowd.

"You're embarrassing me," said the Reagan administration's outspoken advocate in the war against AIDS as he cut off the applause.

Alice Curley, from the Visiting Nurse Association in San Francisco, was one of the first to stand up and cheer. That's because Koop was there when the Coming Home Hospice opened in San Francisco with 15 beds for AIDS patients. "He just inspired us," said Curley.

But it wasn't the same when Vice President Bush took the podium and made his speech.

Bush spoke of his 11-year-old grandson who came home from school asking about AIDS. He spoke of visiting an AIDS patient last April at NIH. "I saw firsthand what this disease can do," he said. "It is brutal . . . unforgiving."

He talked about education: "I don't want the federal government to mandate some insensitive educational program."

Inevitably, Bush turned to the controversial issue of AIDS testing and the administration's plan to expand routine testing programs to detect people infected with the virus.

"As the president said last night, the federal government will soon require testing for prisoners, immigrants and aliens seeking permanent residence -- "

Boos and hisses came from the crowd. Then a few sparse claps. Then more boos.

"We are encouraging the states to offer routine testing for those who seek marriage licenses . . . "

More boos and hisses.

"Of course, any mention of testing must be hurriedly followed by the word 'confidentiality' . . ."

Some laughter rippled through the crowd.

"In closing, let me say we need money for research and treatment. We need education . . ."

Suddenly a shout from a man in the back of the ballroom: "We need a new president!" A few cheers and claps.

The vice president kept right on: "It is our duty to do all we can, not only to find a cure and a vaccine but also to make sure we never create a climate in America, or in any other country, where friend turns against friend . . ."

He ended his remarks as an uncomfortable silence settled over the crowd. "Good luck to all of you . . . We're counting on you."

A few people stood up. Some applause, some boos.

It was then -- after his remarks were over but with the microphones still on -- that Bush turned to Assistant Secretary for Health Robert E. Windom and made his already famous remark: "Who's out there -- some gay group?"