Dr. Margaret Chan, director-general of the World Health Organization (WHO), on Monday called on the governments of countries around the world to remain vigilant against avian influenza and its related pandemic threat.

"The message is straightforward: we must not let down our guard," Chan told a regular session of the U.N. agency's governing Executive Board.

Chan, who took office on Jan. 4, said the whole world had been living under the imminent threat of an influenza pandemic for more than three years.

"These years of experience have taught us just how tenacious this H5N1 virus is in birds," she said.

Chan said the world had made "heroic efforts" to control the deadly H5N1 strain, yet the virus "stays put or comes back again and again."

"As long as the virus continues to circulate in birds, the threat of a pandemic will persist," she said.

As a bird flu expert, Chan warned that the influenza viruses were notoriously sloppy, unstable and capricious, and it was impossible to predict their behavior.

But she also pointed out that the H5N1 strain of avian influenza "is still essentially a disease of birds," as the virus "does not, at present, transmit easily from birds to humans."

The Executive Board, comprised of representatives from 34 WHO member states, implements the decisions and policies of the World Health Assembly, the top decision-making body of the WHO.

The Board also advises the World Health Assembly and generally facilitates its work.

This session of the Board, scheduled to run on Jan. 22-30, will discuss a wide range of issues, including measles, malaria, polio, the prevention and control of chronic diseases, avian and pandemic influenza, and the implementation of international health regulations.