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AIDS researchers and activists were stunned and horrified Friday to learn that prominent AIDS expert Dr. Joep Lange was among those killed aboard a Malaysian Airlines flight shot down over Ukraine on Thursday. It is a shocking loss for the AIDS field, where many researchers, including Lange, have taken on an extra role as advocates for acceptance and treatment of people infected with the virus.

Lange was on his way to a major international AIDS meeting being held in Melbourne, Australia. Others attending the conference said they feared many other AIDS activists and experts headed to the meeting had been aboard the flight as well, although there’s been no official confirmation of names.

President Barack Obama praised them and their work on Friday.

"In this world today, we shouldn't forget that in the midst of conflict and killing, there are people like these, people who are focused on what can be built, rather than what can be destroyed; people who are focused on how they can help people that they've never met; people who define themselves not by what makes them different from other people, but by the humanity that we hold in common," Obama said at a news conference.

"These were men and women who had dedicated their own lives to saving the lives of others and they were taken from us in a senseless act of violence."

Obama also mentioned unconfirmed reports that dozens more delegates to the conference were aboard the flight.

"On board Malaysian Airlines Flight MH-17, there were apparently nearly 100 researchers and advocates traveling to an international conference in Australia dedicated to combating AIDS-HIV," he said.

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"These were men and women who had dedicated their own lives to saving the lives of others and they were taken from us in a senseless act of violence."

“The HIV/AIDS movement has truly lost a giant,” Dr. Chris Beyrer of Johns Hopkins University and president-elect of the International AIDS Society, told reporters at the conference in Melbourne.

“Joep was a giant in this field; a researcher who really ‘got it’ in terms of human rights, equity and justice. He was as much activist as researcher — and that is rare. Incredibly sad,” Mitchell Warren, Executive Director of New York based AVAC: Global Advocacy for HIV Prevention, told NBC News.

Lange helped found the nonprofit PharmAccess Foundation, a Netherlands-based group that worked to get treatment to people infected with HIV in developing countries.

Glenn Thomas, a genial British spokesman for the World Health Organization, was also aboard the flight and en route to the conference. “His twin sister says he died doing what he loved,” WHO said in a statement.

"The deaths of so many committed people working against HIV will be a great loss for the AIDS response."

"The UNAIDS family is in deep shock,” said Dr. Michel Sidibe, executive director of the United Nations’ AIDS arm UNAIDS. "The deaths of so many committed people working against HIV will be a great loss for the AIDS response."

Lange, a Dutch citizen, headed the department of public health at the University of Amsterdam. “Joep was a great source of inspiration for everyone who aimed to contribute to solving the AIDS tragedy in Africa and Asia,” the university said in a statement on its website.

"My good friend Prof. Joep Lange was one of the most creative AIDS researchers, a humanist, and tireless organizer, dedicated to his patients and to defeating AIDS in the poorest countries,” added Dr. Peter Piot, an AIDS researcher and former head of UNAIDS and the International AIDS Society. “Global health and the AIDS response have lost one of their great leaders.”

The University of Amsterdam said Lange’s partner, AIDS communication specialist Jacqueline van Tongeren, was also aboard the flight.

The International AIDS Society, which sponsors the biennial international conference, said it was working to confirm who had been aboard. “In recognition of our colleagues' dedication to the fight against HIV/AIDS, the conference will go ahead as planned and will include opportunities to reflect and remember those we have lost,” it said in a statement.

Grieving colleagues said the deaths were a troubling reminder of the death two other prominent AIDS researchers, Dr. Jonathan Mann and his wife, HIV vaccine expert Dr. Mary Lou Clements-Mann. They were on their way to an AIDS vaccine conference in Geneva when they died aboard Swissair Flight 111, which crashed off the coast of Nova Scotia after leaving New York, killing all 229 people aboard, in 1998.

The conference, which officially starts on Sunday, is the biggest meeting of AIDS activists, researchers and advocates. It’s held in a different city around the world every two years. Former U.S. President Bill Clinton is a regular keynote speaker and is scheduled to address the conference’s 12,000 participants.

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