The World Health Organization issued an emergency global alert yesterday, warning that a mysterious, sometimes fatal pneumonia-like illness posed a worldwide threat after spreading from Asia to Europe and North America.

More than 400 people have been sickened by the disease since the respiratory illness surfaced in China, including at least 150 new cases in the last week alone, officials said. At least nine people have died, and dozens more are seriously ill. The latest death was a nurse who was caring for victims in Hanoi; she died yesterday.

Concern spiked yesterday after the first cases were reported outside Asia. At least eight people have been stricken in two clusters on each side of Canada -- British Columbia and Toronto -- and two have died. In addition, the first case was reported in Europe: A doctor from Singapore was hospitalized in Frankfurt after getting off a plane from New York seriously ill. The unidentified doctor was being treated, and his wife and traveling companion were being kept in isolation.

No cases have been reported in the United States, but federal and state health officials are tracing the movements of the Singapore doctor, who had been attending a medical conference in New York, and one of the Canadian victims, who had recently returned from Atlanta. Officials are concerned that the two may have spread the disease before leaving. The illness appears to have an incubation period of two to seven days.

The federal Centers for Disease Control and Prevention, which dispatched experts to Hanoi earlier in the week and is analyzing blood samples to try to identify the cause, yesterday activated its emergency operations center in Atlanta. The CDC issued a nationwide alert to hospitals and doctors to be on the lookout for cases.

Cases have been reported in Canada, Germany, China, Vietnam, Thailand, the Philippines, Singapore and Indonesia, prompting the WHO to urge nations to take steps to stem further spread.

"This syndrome . . . is now a worldwide health threat," Gro Harlem Brundtland, the WHO's director general, said in the alert. "The world needs to work together to find its cause, cure the sick and stop its spread."

The disease typically begins with flu-like symptoms -- a high fever, coughing, headache, malaise, shortness of breath or other difficulty breathing. It often progresses to pneumonia, with some patients deteriorating to where they require a respirator. So far, the disease has not responded to antibiotics.

"This is a situation that is evolving very quickly," said Health and Human Services Secretary Tommy G. Thompson in a hastily arranged conference call with reporters. "Health officials around the world are taking this situation very seriously."

There is no evidence that the illness is the result of terrorism, but nothing can be ruled out until scientists identify the cause, which has eluded testing in China, the United States, France and Japan, officials said.

Officials said they expected that many more cases may have gone undetected in the Asian countries where the outbreaks began, and still more would likely be detected there and elsewhere around the world.

"It's spreading fast," said WHO spokesman Dick Thompson. "There are already cases from Hanoi to Toronto, and we're just two weeks out."

A global health warning from the WHO is extremely unusual; several officials said they could not remember the last time one had been issued.

The outbreaks appear to have begun in mid-February in the Guangdong province of China, where at least 305 people developed a pneumonia-like illness. Five people died before the outbreak subsided.

The first international alert was triggered when an American businessman who had been living in China arrived in Hanoi from Shanghai severely ill with an unknown respiratory illness. The 48-year-old man, who has not been identified, was hospitalized. After more than two dozen hospital workers became ill, the patient was transferred to a hospital in China. He died there Thursday. The Hanoi hospital was evacuated of everyone except those stricken. Remaining workers started taking extra precautions to protect themselves.

Similar outbreaks were subsequently reported at several hospitals in Hong Kong, prompting the WHO to issue a worldwide advisory on Wednesday warning international travelers.

So far, every test for a known pathogen has come back negative, raising concern that the disease may be caused by a previously unknown virus or other infectious microbe.

"It's always worrisome when you can't put a diagnosis on a disease. You can't predict anything," said David Heymann, head of communicable diseases for the WHO, which has named the disease severe acute respiratory syndrome (SARS).

The disease appears to spread like the flu or pneumonia, through airborne particles or body fluids. But it has only occurred among workers caring for patients, family members, or someone who has had direct contact with "respiratory secretions and body fluids" of a sick person -- not from brief casual contact, the WHO said.

The Frankfurt case involved a doctor from Singapore who had treated two patients with the illness before flying to New York on March 12. He fell ill while in New York and then boarded a plane home Friday. The Singapore Airlines flight was intercepted when it landed for a stopover in Frankfurt. The plane taxied to an isolated runway, and the passengers were removed. The sick man was taken to a clinic in Frankfurt. His wife and a traveling companion were put in isolation. The remaining passengers were reportedly being either temporarily held in quarantine near the airport or continued on to Singapore, where they were to be isolated.

New York health officials are trying to determine exactly where the Singapore doctor traveled, but he is believed to have had "minimal contact with other people" while in New York and attended the conference for a few hours, officials said. An alert was sent to 70 area hospitals.

In Canada, an elderly woman and her 44-year-old son died in Toronto soon after the woman arrived home with other family members from a visit to Hong Kong. Four other relatives -- and possibly a fifth person who came into contact with the family -- have been hospitalized, including a daughter in her thirties who recently had traveled to Atlanta.

"What's significant is that it's now been spread from one person to another in the community. Now that it's out of the hospital setting, it's spreading in communities," Heymann said.

Toronto health officials are trying to alert everyone who came in contact with the victims. A hotline set up for Toronto residents to report possible cases of the illness received 56 calls in the first few hours yesterday, a spokeswoman said.

The victim who had been in Atlanta arrived March 3 and left March 5 after working at a small business, said Georgia State epidemiologist Paul Blake. Health officials have alerted everyone she came into contact with at the business and at the hotel where she stayed, he said. So far, no one has fallen ill. "We may be out of the woods," he said.

Two other cases have been reported in British Columbia, Canada. Both involved people who had recently visited Hong Kong.

Stephen S. Morse, a Columbia University infectious disease expert, said the outbreaks were worrisome because of fears that this could be the beginning of a global flu pandemic. The so-called Spanish flu of 1918 killed millions of people worldwide. New flu viruses historically have first emerged in Asia.

"With globalization, a disease occurring anywhere in the world becomes a concern everywhere," Morse said. "Whenever something unknown appears, and appears to be spreading, it's a cause for concern."

Staff writers Michael Powell

in New York and Al Kamen

in Washington contributed

to this report.

This patient in a Hong Kong hospital is suffering from what WHO calls severe acute respiratory syndrome.