Dr. Lee Jong Wook, director-general of the World Health Organization and the driving force in that agency's effort to expand AIDS treatment to the developing world, died yesterday in Geneva after surgery to remove a blood clot from his brain. The first Korean to head a United Nations agency, Dr. Lee was 61. A 23-year veteran of WHO, Dr. Lee played a key role in eliminating polio from the Western Pacific and organizing the battle against tuberculosis before taking the agency's reins in the aftermath of the SARS epidemic and amid the initial signs of the avian flu crisis. While managing those problems, he triggered the agency's ambitious "3 by 5" campaign to bring at least half of the world's 6.5 million AIDS victims into treatment by the end of last year. Although the effort fell short of its goal, it marked the first serious international effort to bring pricey anti-AIDS drugs to the countries of Africa. Dr. Lee fell ill Saturday at a luncheon marking the beginning of the World Health Assembly in Geneva, complaining of a severe headache and vomiting. He was taken to Cantonal Hospital in Geneva, where surgeons removed a subdural hematoma from his brain. Such hematomas, or clots, can be caused by a blow to the head or by bleeding in the brain caused by an aneurysm. Dr. Lee was not known to have suffered a blow. His death was announced yesterday morning at the official opening of the assembly by Dr. Elena Salgado, Spain's minister of health. Dr. Lee was to have addressed the assembly later in the day. A WHO spokesman said that Dr. Anders Nordstrom, assistant director for general management, will serve as acting director-general until the agency's seventh director-general can be chosen. Tributes poured in immediately. "The world has lost a great man today," said U.N. Secretary-General Kofi Annan. "He was a strong voice for the right of every man, woman and child to health prevention and care, and advocated on behalf of the very poorest people."