For the first time since its 1948 creation, every nation will vote for WHO's new director-general, determining the future orientation and leadership of the controversial, underfunded agency.
"The world is better prepared but not almost well enough", according to Chan, and the WHO still has critical contributions to offer - both seen and unseen. He promised reform of the WHO, which was strongly criticised around the world for failing to respond quickly enough to the Ebola epidemic in west Africa. It said: "Vastly more is expected of World Health Organization while its role is contested and constrained".
Making the agency transparent and accountable.
The victor will succeed Margaret Chan, a former Hong Kong health director who steps down after 10 years on June 30, leaving a mixed legacy after WHO's slow response to West Africa's Ebola epidemic in 2013-2016 which killed 11,300 people.
Nabarro, a WHO insider who has worked for 40 years in worldwide public health, dexcribed himself as a "global candidate". There were calls from some for the abolition of the World Health Organization and the creation of a more effective body.
Dealing with epidemics is only part of the responsibilities of the WHO.
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That sparked controversy as it's a Jewish holy site on disputed territory. Abbas, whose popularity among Palestinians has been declining.
As director-general of WHO, Tedros will be responsible for deciding which medical issues will take priority and when crises like disease outbreaks will be treated as global emergencies. Its mission includes tackling antimicrobial resistance, confronting the growing problem of obesity, diabetes, and reducing maternal and infant mortality during childbirth.
"We know that the next health emergency is not a question of "if" but "when", United States health secretary Tom Price said in Geneva earlier Tuesday. He emerged on top in the two elimination rounds that took place in January and received strong support from high-profile individuals, such as billionaire philanthropist Tony Elumelu; Thomas Frieden, the former director of the U.S. Centers for Disease Control and Prevention; and the late doctor-statistician Hans Rosling. "Whoever becomes the next will only succeed if they make their key human resource decisions on the basis of technical and operational excellence".
"We need WHO to be more effective than it is today", the director of Harvard University's Global Health Institute, Ashish Jha, said at the Swiss Press Club event.
Prior to his election as WHO's next Director-General, Dr Ghebreyesus served as Foreign Affairs Minister of Ethiopia from 2012-2016 and as Health Minister from 2005-2012. Tedros, who will be the first African to be the director general of the World Health Organization, will step into that post in Geneva on July 1. His campaign is thought to have benefited from an endorsement from the African Union and a belief in some quarters that it was Africa's turn to lead the organization.
Some think that politics - China and India - denied another contender, Pakistan's former minister of health Sania Nishtar, a fair go at the position, despite vowing to serve only one term and running on a platform of transparency, reform and collaboration with civil society and humanitarian organizations.