Health authorities around the world are struggling to contain a mystery virus which has so far affected more than 150 people and killed at least nine.
Travellers are taking precautions after a WHO travel alert
The World Health Organisation issued a rare emergency warning over the weekend, declaring the virus "a worldwide health threat", and saying that cases had been reported on three continents.
The WHO warned both travellers and airport staff to be on the look-out for signs of the condition, known as Severe Acute Respiratory Syndrome.
Its symptoms include high fever and breathing difficulties, and it has an incubation period of two to seven days.
On Sunday, the WHO said two deaths had been reported in Canada. The two are thought to be a mother and child returning from Hong Kong, where one victim died last week and 49 people have already been taken ill.
Other cases have been reported in Vietnam, where a nurse died on Sunday and at least 40 other medical workers are infected.
In Germany a doctor suspected of having caught the virus whilst in Singapore is currently in quarantine.
There are also reports of suspected cases of the disease in Slovenia, Taiwan, Thailand, China and the Philippines.
"Until we can get a grip on it, I don't see how it will slow down," said WHO spokesman Dick Thompson.
The outbreak is thought to have begun in the Vietnamese capital Hanoi last month, after an American businessman travelling from Shanghai infected hospital workers before himself dying of the disease.
The flu is thought to be highly contagious
The Vietnamese nurse who died on Sunday is thought to have given him treatment during his time in Hanoi.
David Heyman, the WHO director of communicable diseases, told the BBC said that progress in isolating the cause of the disease was hampered by ignorance about the illness.
"There is no specific antibiotic or anti-viral drug that we can yet recommend," he said.
The East Asian region is the worst affected by the disease.
As well as the cases reported in Hong Kong and Vietnam, authorities in Taiwan have reported three possible cases of the disease.
No figures are available yet from China, but the WHO said the flu-like symptoms of the disease were similar to those of a sickness in the southern province of Guangdong last month which infected 305 people, killing five of them.
"All we can tell you right now is that the disease situation here has been placed well under control," a Chinese government spokeswoman said on Monday.
Japan has sent a team to Vietnam to help investigate the cause of the outbreak.
And Hong Kong's biggest airlines Cathay Pacific has ordered ground staff to turn away passengers who appear sick.
The region is also suffering from a down-turn in tourism as a result of the virus.
"Two big group tours of 40 people from Europe cancelled
this morning and more might be called off this week," said Tran
Trong Kien, chief executive of Buffalo Tours in Hanoi, Vietnam.
But some shopkeepers in Hanoi are reporting brisk trade - with sales of surgical masks and other supplies such as vitamins and nasal medicines at record levels.