News that Malaysia Airlines flight MH17 was shot down in Ukraine sent shock waves around the world. The cracks in an already fragile political landscape began to widen and the senseless loss of innocent life united the world in horror and grief. When it was announced that a group of high-profile Aids researchers travelling to attend an international conference in Melbourne were among the dead, it quickly became clear the impact of this tragedy would be felt far beyond the troubled borders of eastern Europe.

Missing from the delegates at the Aids 2014 symposium was the former International Aids Society president and professor of medicine Joep Lange. As a young doctor in the 1980s, in the early days of the Aids epidemic, Lange led research on the clinical evolution of HIV infection and in the 90s was instrumental in the development of antiretroviral therapy. His ambitions, however, were not limited to furthering scientific knowledge. He made it his mission to widen access to the drugs in developing countries and his loss is likely to be felt most keenly among development workers and activists fighting to improve HIV/Aids care in the world's poorest communities.

Dr Michael Merson, a longtime friend and colleague of Lange, describes the work he was undertaking in east Africa and Thailand as "critical". The founding director of the US-based Duke Global Health Institute says Lange didn't understand why these drugs had to cost so much, particularly for poorer populations: "If you go back to the mid-90s when the antiretroviral drugs first became available, the cost was about $15,000 (£8,800) a year for a patient. Now it is $100 a year. Lange was not the only one who did this, but he was out in front, saying that if we can get Coca-Cola out there why can't we get Aids drugs to the people who need it?"

Merson first met the medical pioneer, who founded the Amsterdam Institute for Global Health and Development, in the 1990s while directing the World Health Organisation's global Aids programme. He hired Lange to lead the unit on clinical research, but after both left WHO they remained friends and continued to collaborate on global health programmes.

Merson mourns losing such an "exceptional" individual. He explains how a lot of people got into the field in the early 80s and bonded during those difficult years when there was no treatment and a lot of discrimination. Lange emerged from those harrowing years as a strong, inspirational leader. Whether you were an activist, scientist, physician, government bureaucrat or working in an NGO, Merson insists you wanted Lange on your side.

Lange's partner and colleague at the institute, Jacqueline van Tongeren, 64, also lost her life in the crash, as well as the director of support at the Female Health Company, Lucie van Mens, and the WHO media coordinator Glenn Thomas who was 49.

Martine de Schutter was another victim of the disaster. For the last year, she has been leading a programme called Bridging the Gap, aimed at providing vital drugs to marginalised groups most vulnerable to HIV/Aids across Africa, Asia and Latin America. According to the executive director of the International HIV/Aids Alliance, Alvaro Bermejo, she was making significant progress in identifying ways to break down the barriers for these groups to access crucial health services. He describes her loss as "a void which is difficult to fill", and her death is a particular blow to development workers in that field.

Bermejo also worked with and speaks of the loss of Pim de Kuijer. The 32-year-old Dutchman was working as an HIV lobbyist, trying to convince the Dutch government to allocate more money to finance Aids programmes in regions where it will make the biggest difference. Bermejo, who knew both de Schutter and De Kuijer well from sharing the same offices in Brussels, Geneva and Washington, claims his knowledge of the Dutch parliament was incredibly detailed and believes it will be difficult to replace him.

The Aids movement, however, is sadly used to losing leaders. Some have died as a result of the virus itself, while others have met a similar tragic fate to those travelling on MH17. In 1998, Jonathan Mann, former director of the WHO's international Aids programme, died when a Swissair flight from Geneva to New York came down over Nova Scotia. Replacing leaders, Bermejo says, is something that the Aids movement has had to do more than any other.

Despite the grief and disbelief one of the major strengths of the Aids community is the spirit of collaboration. It is the ability to work together on a global scale that Bermejo hopes will push the movement forwards after this tragedy.

He says: "Pim and Martine were really great examples, not just of researchers and implementers, but of how the power of alliances and collaboration can take you beyond what one organisation can do.

"That has to be part of their legacy – that commitment to continue working in partnerships across organisations to bring about the end of the Aids epidemic."

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