In its first warning that suggests the disease can be spread on airplanes, the World Health Organization said passengers with flu-like symptoms or who may have been exposed to severe acute respiratory syndrome, SARS, should not be allowed to fly.
"If the passengers are sick, health workers will be recommending to the airline that they not board the plane," said Dr. David Heymann, WHO's infectious diseases chief.
The WHO advice - it can only make recommendations to governments - is directed at flights leaving cities where the disease is spreading locally: Toronto; Singapore; Hanoi, Vietnam; Hong Kong; Taiwan; Beijing, Shanghai and the Chinese province of Guangdong, where the earliest cases of SARS occurred.
While suspected cases have been reported in more than a dozen countries, the illness is not considered to be spreading in most communities. So far, the WHO says 1,408 people have fallen ill with SARS and 53 people have died; that doesn't include a death Thursday that Hong Kong officials were reporting.
The United States has 51 suspected cases, said Dr. Julie Gerberding, head of the U.S. Centers for Disease Control and Prevention.
The death rate has remained around 4 percent since the outbreak began, experts said. There have been three deaths in Canada and none in the United States.
Thursday's advice from the WHO was the latest broad attempt at slowing the spread.
"The recommendation still remains that there should be no interruption in travel or trade, but we're shifting a little bit more responsibility to countries where there are these infected areas," Heymann said.
However, the CDC's Gerberding encouraged Americans to defer vacations to Asia if they were able to do so.
"This is now a global epidemic and potentially a global pandemic," if it's not quickly brought under control, she said.
The WHO has teams of infectious disease experts in the affected countries offering support. But some places continue to have problems containing the disease, which apparently got its start last winter in Guangdong province in China. It is believed to be a virus and there is no treatment for it, although medicines are being tested.
In Hong Kong, where at least 10 people have died, the government said Thursday it would quarantine more than 1,000 people and close its schools. Weekend concerts by the Rolling Stones were postponed.
In Ontario, Canada's most populous province, health authorities declared a state of emergency and called for a 10-day quarantine of people who had visited a hospital where the outbreak spread - a number that could be in the thousands.
Ontario health officials said Thursday they had bought all the high-grade surgical masks available in Canada to protect medical workers from the disease.
"One hundred thousand were delivered yesterday, 200,000 will be delivered today and the last 40,000 were purchased early this morning," said Dr. Colin DCunha, Ontarios Commissioner of Public Health.
Canadian Health Minister Anne McLellan said she is taking the WHO screening recommendation "very seriously" and her office has already started working with the airlines.
Health officials already knew the disease had spread beyond Asia by international air travel on March 15, when the Canadian cases turned up. But at the time it was not clear whether those people were sick on the plane or whether they got sick after coming home.
"Now we know that there are people who are traveling when they are sick," Heymann said. "There have been more and more cases who have traveled on airplanes and we aren't yet 100 percent sure of how this disease is spreading."
He said experts remain convinced that the infection is spread only by very close contact through coughing and sneezing.
However, there may be people with a less severe bout of the illness, or some who are infected but showing no symptoms, who are transmitting it, Heymann said.
The new airline advisory also recommends a tougher approach on board if the flight crew detects someone becoming noticeably ill with fever and breathing problems.
"The passenger should be as isolated as possible from others and should be asked to wear a protective mask," said WHO spokesman Dick Thompson.
Sick passengers should also be assigned their own toilets on board, he added. The aircraft captain should radio ahead to the airport to alert health authorities and the passenger should be quarantined, the new recommendations say.
WHO is not suggesting all air travelers wear masks, as some in Asia have done recently.
Heymann said that although new suspected SARS cases are added to the official list every day, the disease is not spreading any faster than anticipated. "I think the control measures that are going on are checking the spread of this," he said.
It appeared Thursday that scientists were zeroing in more precisely on the cause of the illness, which several labs report is a new type of coronavirus. That virus is second only to rhinovirus as the cause of the common cold.
However, there is some evidence that a second germ, the paramyxovirus, could also be at play, perhaps in tandem with the coronavirus, WHO experts said.
"I think we're closer to the real answer, but the trump card is what is paramyxovirus doing? Is it all related and that's what has to be sorted out," Heymann said.
Although there is no specific recommended treatment for the ailment, most patients seem to get better with normal hospital care. About 10 percent fare badly, but many of those have other illness such as diabetes or heart disease, which complicates their care.
By Emma Ross