Dr Lee Jong-wook, the director-general of the World Health Organisation, died unexpectedly this morning after suffering a blood clot on the brain. He was 61.

Dr Lee, originally from South Korea, had been due to address the WHO's annual meeting, which opens in Geneva today. But he fell ill at an official function on Saturday, and did not recover from emergency surgery.

Colleagues said today that they were devastated by his sudden death, and praised him as an exceptional person and an exceptional director-general. Flags flew at half-mast on the UN building, and the start of the meeting was delayed for half an hour.

World leaders, including Kofi Annan, sent their condolences to his wife and son.

Anders Nordstrom of Sweden, whom Lee had named to take over in an emergency, will serve as acting director-general.

Dr Lee took over as head of the WHO in 2003, after the retirement of Gro Harlem Brundtland, a former Norwegian prime minister, who had re-energised the once disorganised and disillusioned organisation.

He had worked for the WHO for 23 years, leading a widely praised campaign against Tuberculosis, and was the only insider in the leadership race.

Fears that he might lack the political astuteness of some of his rival candidates were disproved early in the race when it emerged that he had persuaded 53 members of the US Congress to write to the US Secretary of State Colin Powell and Tommy Thompson, the US health secretary, backing his candidacy.

He believed that his time in office would be defined by the fight against HIV-Aids, particularly in the hardest-hit poor countries, but in fact it was other infectious diseases which came to dominate his mandate.

He took over at the height of the outbreak of Sars, or severe acute respiratory syndrome, in Asia. After the threat from Sars was contained, the WHO turned its attention to bird flu amid fears that the virus could mutate into a strain easily transmitted among people.

For the last two years Dr Lee has been at the forefront of efforts to tackle the spread of bird flu through Asia, Europe and Africa and its potential for causing a human influenza pandemic.

"We know another pandemic is inevitable," Lee told a meeting of experts in 2004, to discuss preparations for the emergence of a new strain of flu virus, expected to come from a change in bird flu.

"And when this happens, we also know that we are unlikely to have enough drugs, vaccines, health care workers and hospital capacity to cope in an ideal way. So we must act wisely."

The agency encouraged experts make continency plans. It built up a reserve of anti-viral medicines and encouraged vaccine research.

At a global donors’ meeting in Beijing in January, $1.9 billion was pledged to the fight against bird flu and preparations for a potential pandemic.

Today the WHO issued a statement, which said: "The sudden loss of our leader, colleague and friend, is devastating. All of the staff of the World Health Organisation extend their most sincere condolences to Dr Lee’s family."

"This is a very sad bit of news," Elena Salgado, Spain’s health minister and president of the World Health Assembly, said at the opening of the annual meeting of the 192 WHO members. "He was an exceptional person and an exceptional director general."

"I knew Dr Lee for many years and had nothing but the highest admiration for his commitment to global public health," said Dr Shigeru Omi, WHO’s regional director for the Western Pacific. "He will be sorely missed."

Explore World News

• World Agenda

• Iraq News

• US & Americas News

• Europe News

• Middle East News

• Afghanistan News

• Asia News

• Africa News

• Ireland News

Times Recommends

• A doctor, a daughter and a man called Johnny
• Mona Lisa's smile may have belied ill health
• ‘Parasite economy’ lies behind Greek tragedy