At least six passengers who died on board the crashed Malaysia Airlines plane were travelling to a major international Aids conference.
They include Prof Joep Lange - a prominent and popular researcher and a former president of the International Aids Society (IAS).
IAS said they would have "truly lost a giant".
Delegates who have already arrived at the conference in Australia said they were in "total shock".
Early reports suggested that at least 100 delegates had been killed, but that figure has now been revised to six.
They were named by the IAS as Pim de Kuijer, Lucie van Mens, Maria Adriana de Schutter, Glenn Thomas, Joep Lange and his partner Jacqueline van Tongeren.
"The extent of our loss is hard to comprehend or express," said IAS President Francoise Barre-Sinoussi. "We grieve alongside all of those throughout the world who have lost friends and family in this senseless tragedy."
More than 14,000 scientists, campaigners and politicians are meeting at the Aids 2014 conference in Melbourne.
Joep Lange was a professor of medicine at the University of Amsterdam and has been involved in HIV research since the virus emerged in the 1980s.
He trialled antiretroviral therapies, which have transformed HIV into a manageable disease.
He also worked on preventing the virus passing from mother to child during pregnancy and labour.
Prof Lange is described as a leader in his field and between 2002 and 2004 was president of the International Aids Society.
Prof Peter Riess, who also worked at the University of Amsterdam, told the BBC: "Joep was a close colleague and friend of mine. Everyone here in Melbourne is in total shock at what happened.
"In the early eighties when this strange new disease hit Amsterdam, both Joep and I were training at the time and were confronted with this new disease which then went on to shape our scientific and medical careers.
"He's been really involved from the very beginning."
The plane, carrying 298 people, crashed in rebel-held territory in eastern Ukraine on Thursday.
Dr Jeremy Farrar who leads the Wellcome Trust medical research charity said he was "deeply saddened" by the deaths on flight MH17.
"Joep was a great clinical scientist, and a great friend of the Wellcome Trust who has long been a valued adviser.
"He was also a personal friend. He is a great loss to global health research."
Zoya Shabarova, an adviser for the Aids Healthcare Foundation, was flying to the conference via Hong Kong.
She told the BBC: "It's a really terrible loss, unbelievable, he was a person with such a big heart. My colleagues and I, we can't comprehend this, it's a terrible loss to the HIV programme, the people, the patients.
"I want to send deep condolences to his family and all those on the flight."
Speaking about the loss of another delegate, Glenn Thomas, of the World Health Organization, WHO spokeswoman Fadela Chaib said: "For the time being we would like to give his family time to grieve.
"We have lost a wonderful person and a great professional. Our hearts are broken. We are all in shock."
"The global HIV community is a close community - standing up for non-violence, dignity and human rights. People will be devastated - but I am sure everyone will pull together and continue to fight the AIDS epidemic."