Nine Chinese tourists appear to have caught a dangerous new respiratory illness by flying on a plane from Hong Kong to Beijing with a man who was sick, officials reported yesterday.

The infections, if confirmed, would mark the first known incident in which someone was stricken with the disease by sharing a plane ride with an infected person.

The report heightened concern about how easy it is to contract the illness, raising questions about whether new restrictions should be imposed on air travel to the most severely affected countries to stem further spread of the disease, known as severe acute respiratory syndrome (SARS).

After a lengthy international conference call, the World Health Organization (WHO) decided not to recommend restricting travel.

"Literally thousands of people have been on planes where known and sick SARS patients have been," WHO spokesman Dick Thompson said. "We haven't had one case out of thousands of possible contacts of SARS transmission on a plane."

The Canadian government, however, advised its citizens to "defer all travel until further notice" to the hardest-hit parts of Asia -- Hong Kong, Hanoi, Singapore and the Guangdong Province in southern China, where the disease is believed to have first emerged.

Nineteen cases and three deaths have been reported in Canada, but officials in Toronto said yesterday that at least 55 people may have become infected, including 25 health care workers.

So far, the majority of cases worldwide have occurred among people who recently traveled in Asia, medical workers who cared for them or family members.

The Chinese airplane infections apparently occurred when a 73-year-old man who had visited his sick brother at a Hong Kong hospital boarded a March 15 flight to Beijing even though he had begun to feel ill. The other passengers included 35 members of a tour group plus a travel agent who organized the trip.

A couple who had sat near the sick man, both age 44, developed symptoms within a few days. After the couple returned home, they were admitted to a Hong Kong hospital, where they were in stable condition.

After being notified of the cases Sunday , health officials contacted other members of the tour.

One man and two women, ages 27 to 50, had developed symptoms and were admitted to hospitals for treatment. They were in stable condition.

Six other members of the tour -- three men and three women, ages 32 to 61 -- were hospitalized for surveillance because they had developed coughs. Two of the men and two of the women, ages 32 to 51, were confirmed to have been infected and were being treated. The fifth person, a 61-year-old man, was under observation. The sixth was discharged.

Health officials asked any other passengers on the two flights to contact them, and advised the airline to disinfect the planes.

WHO remained cautious.

"That case really needs to be looked at hard, and that's what we're doing," Thompson said. "We have thousands of potential contacts on other flights and just don't see SARS patients coming out of that group. After very long consultation and discussion and review of the data, we're just not changing our travel recommendations."

Nearly 500 cases of SARS and 17 deaths have been reported in 12 countries in Asia, Europe and North America since WHO issued an unusual global health alert last month. By far the hardest hit has been Hong Kong, where 286 cases, including 26 new cases in the previous 24 hours, and 10 deaths have been reported. U.S. health officials are investigating 40 possible cases in 18 states, including three in Virginia, but no deaths have been reported.

In Toronto, officials have ordered 25 health care workers who may be "symptomatic" to stay home under quarantine for 10 days until they are possibly no longer infectious. Their families were also ordered to remain under quarantine.

"We are taking steps to contain the spread of SARS," said Colin D'Cunha, Ontario's commissioner of public health. At the Scarborough Hospital-Grace Division, where the three fatal cases were treated, officials have closed the emergency room to admissions, nonurgent services and visitors.

A 76-year-old Toronto man died Friday after he shared a hospital room with a man with the illness. Health officials warned family members to wear masks at the man's funeral. The family members reportedly also drove themselves to the funeral because limo drivers refused to drive them, fearing any contact would make them sick.

Sheela Basrur, Toronto's medical officer of health, said that there are 18 probable cases of the syndrome in Toronto, five suspected cases and 25 people who have symptoms. Most of the 25 are health care workers, including nurses, doctors, paramedics, a firefighter and technicians who had contact with patients before officials knew they were dealing with an unknown illness.

Despite the jump in possible cases, officials urged residents to remain calm.

"I don't think anybody has to be concerned about going to the movies or traveling on a subway," Basrur said. "The cases we are monitoring now are the result of unprotected contact with initial cases before we were aware of SARS. Health care staff across the province are alert to the virus, and are able to protect themselves and their patients from further infection."

The disease is believed to have emerged in November in southern China, where at least 300 cases and five deaths have been reported. WHO officials have been pressing leaders in China for more information about whether the outbreak is continuing there or spreading elsewhere on the mainland.

Scientists are trying to determine how two previously unknown viruses that have been detected in patients, including one that usually causes the common cold, may be involved in the new disease.

Stein reported from Washington; Brown reported from Toronto.

Masks help protect a mother and child in Hong Kong against a mysterious respiratory illness.