In a rare "emergency travel advisory," the health agency said it has received more than 150 reports of what it called "Severe Acute Respiratory Syndrome" in the past week alone, mostly in Southeast Asia. At least three people have died -- an American businessman and two people who arrived in Canada recently from Hong Kong.

"Health officials around the world are taking this situation very seriously," US Health and Human Services Secretary Tommy Thompson said Saturday.

The department was "applying a full-court press to learn more about this outbreak and how it might impact on the United States," Thompson said.

While no formal travel restrictions are in place, US health officials said travelers may wish to postpone nonessential trips to countries at risk. Health officials are preparing to issue an alert for passengers returning from countries where SARS has been reported.

The growing list of countries reporting cases of the illness include Hong Kong, Indonesia, Philippines, Singapore, Thailand and Vietnam. Most cases involve medical workers.

No cases have been identified in the United States, health officials said. But a doctor believed to be infected was taken off a New York-to-Singapore flight in Germany on Saturday and quarantined.

Two people traveling with him -- his wife and another doctor -- also were being held for observation at the Wolfgang Goethe University Clinic in Frankfurt, Germany.

In New York, health authorities put hospitals on alert.

Also, a man traveling from Atlanta to Canada is "reported to have developed some respiratory symptoms," said Dr. Julie Gerberding, director of the federal Centers for Disease Control and Prevention in Atlanta.

Doctors do not know what causes the illness -- even whether it is a bacteria or virus. Gerberding said doctors are unsure whether antibiotics or antiviral drugs have an effect on the disease since they have not been consistently used in the areas with the most cases.

The potentially fatal illness is believed to spread "person to person" and have an incubation period of two to seven days, Gerberding said.

"There is no evidence to suggest that this can be spread through brief contact or assemblages of large people," Gerberding said.

When asked whether the illness could be caused by bio-terrorism, Gerberding said, "We have an open mind and will be keeping an open mind as we go forward."

WHO spokesman Dick Thompson said in Geneva, "Until we can get a grip on it, I don't see how it will slow down. People are not responding to antibiotics or antivirals. It's a highly contagious disease and it's moving around by jet. It's bad."

One of the most severe outbreaks has been in Hanoi. A CDC team of epidemiologists flew to the Vietnamese capital Saturday and gathered samples from people who may be infected. The samples were immediately flown to Atlanta for laboratory testing.

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

The Singapore doctor began suffering symptoms while in New York, said Dr. Angela Wirtz, a health official in the German state of Hessen, where the patient is being treated.

The man recently attended a New York conference, but it was not immediately known exactly when he was in the city, the nature of the meeting or which airline he used.

There was concern the doctor may have infected others on board.

Another 155 passengers who deplaned in Frankfurt were quarantined at the airport. German nationals were released while passengers in transit to other cities in Europe were awaiting travel permission from those countries, German health officials said. They did not give a breakdown of number of travelers or destinations.

Eighty-five people bound for Singapore and the plane's 20-member crew continued their journey but were to be quarantined upon arrival, health officials said.

The WHO advisory urged travelers who may have come in contact with someone infected to watch for symptoms such as high fever, coughing and shortness of breath.

SARS also may be associated with headache, muscular stiffness, loss of appetite, confusion, rash and diarrhea.

The advisory did not call for restrictions on travel to any destination but said people who suspect they may have the illness should seek medical attention and not travel until they recover.

WHO officials said they could not remember issuing such a travel advisory before.

The illness is a "worldwide health threat," Brundtland said. "The world needs to work together to find its cause, cure the sick, and stop its spread."

In Atlanta, the CDC emergency operations center has been activated and its staff is working around the clock. US health officials are in contact with health officials in China, where cases have been reported for at least several weeks.

"We are doing everything humanly possible to learn what is causing this outbreak," Thompson said from O'Hare International Airport in Chicago, before flying to Washington.

The Hanoi outbreak started after an American businessman traveling from Shanghai via Hong Kong apparently infected up to 30 hospital workers, five of whom now are in critical condition. The unidentified US citizen was evacuated and died in Hong Kong.

In southern China's Guangdong province, an illness has in recent months killed five people and sickened more than 300 with pneumonia.

In Canada, Toronto Public Health officials said a woman died March 5 and her adult son died March 13 after arriving recently from Hong Kong. Four of their relatives were hospitalized.

The illness also might have emerged in British Columbia, Canada, where one person was in intensive care at a Vancouver hospital and another person has recovered, Toronto health officials said.

Toronto has established a hot line for people who fear they have the illness.

(China Daily March 16, 2003)