Clinton called the news sickening and awful.

As many as 100 people who were on the plane shot down over Ukraine yesterday may have been AIDS researchers and advocates heading to the International AIDS Conference. That number is according to President Obama. It is not clear yet who all was on the plane; a full list has not been released. But it's been widely reported that among them was Dr. Joep Lange, a pioneering HIV/AIDS researcher. He was "the architect and principal investigator of several pivotal trials on antiretroviral therapy" beginning way back in 1983, according to his official bio. What does that mean? In layman's terms, according to Dr. Peter Shalit, who practices medicine in Seattle and treats many patients with HIV, studies conducted by Dr. Lange led directly to the drugs used today to suppress the HIV virus—the only drugs that turned out to be effective at keeping people alive.

"The community of HIV researchers is a tight group," Dr. Shalit said. "There's a lot of comraderie because everyone's on the same side. So it's really horrible," he said, adding that it wouldn't just have been researchers but also their assistants, communications staff, nurses, etc. "I never met Joep Lange but a lot of people I know know him. That's sort of the way it all works. There's a relatively small concentrated group of doctors that work on HIV. We know of each other by name if not personally." Dr. Shalit personally attended the International AIDS Conference last year—it's one of the three or four most important AIDS meetings globally each year.

"It's so tragic. I couldn't believe it when I saw it," said Dr. Brian Wood, who teaches at the University of Washington and is a provider at Harborview's Madison Clinic. "I think everyone in the HIV community is pretty shocked and saddened by it. It's a pretty big setback to lose not just that many researchers but researchers who were involved in the beginning and pivotal in the early days. It's a really tragic loss and a huge setback for the field." Asked if he knew anyone on the plane, Dr. Wood said, "I don't know if I know anyone" connected with UW or Harborview who was on the plane.

Michael Nank, a spokesperson for Fred Hutchison Cancer Research Center, said, "We've got several people already on the ground in Melbourne getting ready for the conference," but no one who was on the plane, he doesn't think. "Again, no one's sure." In a statement, Fred Hutch's Dr. Larry Corey called it "so ironic that people who dedicate their lives to helping others are taken from us in such a senseless way."

It wasn't just Dr. Lange's work at the vanguard of developing retrovirals that made him such an internationally revered figure, Dr. Shalit told me. Once that work was accomplished, Dr. Lange "was an early advocate of getting antiretrovirals into Africa—not just the drugs but the infrastructure you need in terms of delivery and refrigeration. He advocated for infrascruture that would get drugs to people. You can't just ship them and drop them off. You have to make sure they get to the people that need them."

More than one million people die of AIDS in Africa every year.

Bill Clinton has described the news about the plane as "sickening" and "awful." He said, "Thinking about those people being knocked out of the sky is pretty tough."

Dr. Shalit said, "It's very distressing. The people on that plane, what did they do? They're just trying to get from point A to point B."

Colleagues of Dr. Lange's have described him as someone who "changed the course of humanity." His partner, Jacqueline van Tongeren, is believed to have been on the flight, too.

UPDATE: The number of AIDS researchers on the plane has been revised to six, and a full flight manifest has been released.