According to the World Health Organisation (WHO), 487 cases of the disease have been reported worldwide. Most are in the Far East, but there are four cases in Britain and Ireland, nine in the rest of Europe and 50 in north America.
Singapore, where one person has died so far, sent half a million pupils home yesterday. All its schools are to be closed until April 6 and 700 people have been put in quarantine.
In Hong Kong, where many people are wearing protective masks, about 60 schools have been reported closed. The Education and Manpower Bureau would confirm only five closures - after eight pupils were infected by sick relatives or health care workers - but officials acknowledged that some had closed at their own initiative. About 2,000 schools are being disinfected.
On Wednesday a school in Toronto, Canada, shut after three children developed high temperatures. The 340-pupil David Lewis school is near a hospital where the first of 18 probable cases was reported. Another 25 people, mainly hospital workers, are showing symptoms and remain in quarantine.
"On purely medical grounds, there are currently no strong reasons for closing all schools," said Teo Chee Hean, Singapore's education minister, on Wednesday. "However, principals and general practitioners have reported that parents continue to be concerned about the risk to their children in schools."
Dr David Heymann of the WHO said the two countries of most concern were Hong Kong and China. He said that experts needed to establish urgently whether the cases in schools were occurring randomly or were "cases which sat next to students who were sick".
He said in Hong Kong it was known that people who had been in schools had become sick, "so this is where the efforts must be now intensified, in addition to better understanding what is going on in China".