GENEVA — World Health Organization chief Dr. Margaret Chan has earned a reputation for ruthlessness when it comes to battling viruses.

Her decision to slaughter 1.5 million birds in three days to stamp out the first human outbreak of bird flu in Hong Kong may well have stopped the virus from mutating into a deadly strain.

The world may need similar decisive action now, with the swine flu outbreak that originated in Mexico threatening to turn into a pandemic. Some health experts say Chan is definitely up to the task.

"I couldn't imagine the world in better hands," said Dr. Jim Yong Kim, incoming president of Dartmouth College, who recruited Chan when he worked at WHO. "This is someone who is cool under fire."

Dr. Jeffrey Koplan, an ex-director of the U.S. Centers for Disease Control and Prevention, called Chan the "perfect" WHO chief for the current crisis.

"She's a good leader in general, and she's a good leader in a crisis," Koplan said.

In her time at WHO, Chan has become known as a direct manager, who can push through politically sensitive issues and occasionally disarm colleagues with a song.

Forced into the spotlight yesterday as the world's attention focused on the swine flu infections on two continents, Chan's words matched the authority of her actions.

"It really is all of humanity that is under threat during a pandemic," she told a news conference in Geneva, announcing that WHO was raising its pandemic alert level to Phase 5 — one step short of a global epidemic.

Chan is known for her diplomacy.

"Avoiding open fights is kind of the sine qua non of working for an international organization," Koplan said. "But when it comes to protecting public health, I don't think she would hesitate for second to do whatever is necessary, including being at odds with a WHO member state."

Still, Chan has been criticized for staying neutral in an international dispute over Indonesia's refusal to share bird flu samples with the scientific community. Not being able to track Indonesia's bird flu outbreak leaves a gaping hole in experts' ability to see how bird flu is evolving, but Chan has never openly questioned Indonesia's actions.

The physician rose to prominence in 1997 when the H5N1 bird flu virus killed six people in Hong Kong. As the region's health director, she immediately ordered the slaughter of Hong Kong's entire poultry population — an action credited with possibly averting a pandemic.

In 2003, Hong Kong was one of the centers of the global outbreak of SARS, severe acute respiratory syndrome. The illness infected more than 8,000 people and killed 774 — including 299 in Hong Kong — but Chan was praised for her role in containing the outbreak and caught the attention of the late former WHO chief Lee Jong-wook.

His team recruited Chan to Geneva, where she headed the department responsible for fighting infectious disease threats.

She became WHO's director general after Lee's sudden death in 2006.

Chan, born in Hong Kong in 1947, received her medical degree from the University of Western Ontario in Canada and joined the Hong Kong Health Department in 1978.

She specialized in maternal and child health before becoming Hong Kong's deputy health chief in 1992, then director in 1994.

Chan is married and has a son.

"We must not let our guard down," she said, speaking about the possibility of a pandemic on her first day at the helm of WHO in 2007. "We must maintain our vigilance."

Two weeks later, she said the world was "years away from control in the agricultural sector," and that it was "impossible to predict" the behavior of the "notoriously sloppy, unstable, and capricious" influenza virus.

She was right.