The number of people believed to be infected with the Sars virus in the Canadian city of Toronto has jumped from 12 to at least 33.
The dramatic rise has been largely attributed to a change in the way health officials define cases of Sars.
Meanwhile, the number of people under quarantine for possible exposure to Sars in Toronto has more than doubled, with more than 7,000 people now in isolation.
Thirty people have died from Severe Acute Respiratory Syndrome - or Sars - in the Toronto area, making it the worst-hit place outside Asia.
China, where the pneumonia-like illness originated, reported seven new Sars cases and one fatality on Friday.
More than 300 people have died from the virus in China since the first suspected case there in November, 2002.
Singapore, on the other hand, has been declared Sars-free with the World Health Organisation praising its 'exemplary' handling of the disease.
Fears of more cases
Toronto health officials have brought the way they define Sars cases in line with WHO recommendations.
Previously, officials diagnosed Sars based on deteriorating breathing problems; now the definition has been broadened to include patients with pneumonia, the cause of which cannot be explained.
Health officials had been accused of deliberately underestimating the number of cases so as to avoid another travel warning from the WHO.
Earlier this week, the WHO placed Toronto back on its list of Sars-affected places, 12 days after it was taken off.
The provincial public safety officer, Dr Jim Young, said the number of Sars cases in Toronto could reach at least 70.
Quarantine broken
Correspondents say that although officials continue to insist that Sars has been contained in Toronto, there have been some worrying developments.
Two students who were supposed to be in voluntary quarantine are known to have broken their isolation.
In the west-coast city of Vancouver, two new suspected cases of Sars have been reported, unrelated to the Toronto outbreaks.
The patients had recently travelled to Asia, where the virus originated.
