The World Health Organization yesterday called for new travel restrictions on all flights leaving locales hard hit by the mysterious illness known as SARS: China, Vietnam, Singapore, Hong Kong and Toronto.

The agency advised that passengers should be screened for any signs of severe acute respiratory syndrome and asked whether they have recently been in contact with anyone with the disease, and that those who appear symptomatic should not be allowed to fly.

A passenger who becomes ill during a flight "can be isolated from other passengers and crew, can wear a mask, and can then be assessed when they arrive at their destination" for signs of SARS, WHO's Dr. David Heymann said in a news briefing in Geneva.

He added that people living in areas with identified SARS cases should avoid people who are coughing. "So that if you're standing next to someone on an escalator who's coughing, you ought to move back two or three steps," he said. "... People who have cough and fever have just as much responsibility to go to a health worker as do people who are standing near them to protect themselves."

These WHO guidelines come on the heels of heightened restrictions on patients and their contacts in Singapore, Hong Kong and Canada. Singapore and Toronto have imposed quarantines on such individuals, and both countries have asked citizens to defer travel to China or Hong Kong. The Rolling Stones canceled this weekend's Hong Kong concerts.

So far, the U.S. Centers for Disease Control and Prevention sees no need to impose travel restrictions or quarantine. But CDC director Dr. Julie Gerberding encouraged Americans to defer vacations to Asia if possible. As of yesterday, 51 U.S. cases have been identified. No deaths have been reported.

Worldwide the number of reported cases hit 1,408 yesterday, with 53 deaths. Most of the increase was reported in China, where the government says cases extend beyond Guangdong province to interior regions around Beijing and Shanxi.

Global health experts neared agreement that a type of virus responsible for some common colds is the likely cause of the outbreak, now in 14 countries. And most of the world's cases can be traced to a single hotel in Hong Kong.

Though at least three types of viruses have been theoretically linked to SARS, "all indications point to coronaviruses," Dr. James Hughes, director of the national Center for Infectious Diseases, said in a news briefing. And Dr. Klaus Stohr, who is running the WHO effort to find the cause, said yesterday, "We believe that the coronavirus is the major causative agent. Now the data that is forthcoming from very many different laboratories which investigated samples from Vietnam, from Hong Kong, from Singapore, from Germany, and from Canada - all these laboratories are consistently finding coronaviruses in those patients."

Though most common colds are caused by a family of microbes called rhinoviruses, a subset is caused by coronaviruses. Scientists at Hong Kong University have invented two tests for the virus - one that uses blood samples, the other, saliva. So far, the tests appear reliable, Stohr said.

The CDC, in collaboration with colleagues in Asia, yesterday published a road map of the epidemic, showing that most known cases after Feb. 15 can be linked to the Metropole Hotel in Hong Kong. The hotel outbreak started when a man from Guangdong, China, traveled to Hong Kong and stayed on the hotel's ninth floor. He died on Feb. 22. That first case spawned many of the subsequent Hong Kong cases, mostly among health care workers. As of yesterday morning Hong Kong had 367 cases, 10 of them fatal.

The disease appears to have spread when another hotel guest, a 47-year-old Chinese-American businessman, traveled to Hanoi, where he became ill. He was then evacuated to Hong Kong, where he died on March 12. Fifty-nine Vietnamese cases have been linked to Cheng's treatment at a Hanoi Hospital, four of them fatal. One of the Vietnamese patients went to Thailand, spawning three cases there.

Three travelers from Singapore who stayed at the Metropole Hotel are believed to have sparked 70 cases when they returned home, mostly in health care workers.

Another visitor to the Metropole was a Chinese-Canadian, who took ill after flying home to Toronto. Canada now has 28 cases, three of them fatal, most believed linked to that first traveler.