Geneva ? Dr. Lee Jong-wook, who spearheaded the World Health Organization’s successive battles against SARS and bird flu and was the first South Korean to head a U.N. agency, died Monday following surgery for a blood clot on the brain. He was 61.

Lee fell ill Saturday while attending a function in Geneva and underwent surgery later that day, the agency said.

Anders Nordstrom of Sweden will take over as WHO’s acting director-general.

U.N. Secretary-General Kofi Annan called Lee “a valuable leader to WHO staff the world over.”

“This sudden loss of a leader, colleague and friend is truly devastating,” he said.

Lee took over as director-general of WHO in 2003 as the agency battled the SARS outbreak in Asia. After that threat was contained, WHO turned its attention to bird flu amid fears the virus could mutate into a strain easily transmitted among people.

The agency oversaw a number of preparatory meetings as experts developed their plans to tackle the H5N1 strain. WHO also built up a reserve of antiviral medicine 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 to prepare for a potential pandemic.

Lee worked for WHO for 23 years, including time served in regional posts. He was the first South Korean to head a U.N. agency, after winning praise for his low-key but efficient management style as head of the agency’s tuberculosis program.

Time magazine named Lee one of the world’s 100 most influential people in 2004.

U.S. Secretary of Health and Human Services Mike Leavitt, who traveled with Lee last year to southeast Asia to learn about a possible pandemic, paid tribute to his leadership.

“During the course of our travels, Dr. Lee shared with me how he was as a young boy from the war-torn country of Korea,” Leavitt said.

“He spoke with me of three difficult and arduous months when he and his mother walked mile after mile after mile in search of his father, who was during that cold winter in exile. Dr. Lee experienced hardship at a very early age, and my sense is it was the reason that he chose to devote himself to public service,” he added.

Lee “was an exceptional person and an exceptional director general,” said Elena Salgado, Spain’s health minister and president of the World Health Assembly, at the opening of the annual meeting of the 192 members of WHO. Flags flew at half-staff on the U.N. building, where the meeting was taking place.

Lee initially said he wanted to improve international monitoring to help tackle outbreaks of diseases like SARS and that his mandate would be defined by the fight against HIV/AIDS, particularly in hardest-hit poorer countries.

But his time in office came to be dominated by the spread of bird flu through Asia, Europe and Africa and its potential for causing a human flu pandemic.

“We know another pandemic is inevitable,” Lee told a 2004 meeting of experts. “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.”

Lee was elected by WHO’s executive committee in 2003 to replace Gro Harlem Brundtland, a former Norwegian prime minister who stepped down after transforming WHO from a disillusioned and badly managed organization to a high-profile agency that has put health firmly on the global political agenda.

Lee, a tuberculosis expert, had previously run WHO’s Stop TB program.

He was the only WHO insider in the race for the top job in 2003 and the only candidate never to have held a ministerial or top U.N. post.

Although initially regarded as a political lightweight, he showed his acumen by persuading 53 members of the U.S. Congress to write to then-Secretary of State Colin Powell and Tommy Thompson, the health secretary, backing his candidacy.

Lee is survived by his wife and son.