A key official in Toronto's SARS containment effort says it's important hospitals keep their guard up against the mysterious illness in spite of the World Health Organization's decision to remove the city from the affected list.

Wednesday's decision by the WHO means Toronto has gone the mandatory 20 days, two full incubation periods, without reporting a single new case of the virus.

Dr. Donald Low, a leading SARS expert and chief microbiologist at Mount Sinai Hospital, told CTV's Canada AM it's important to remain vigilant.

"We're still concerned. We want to make sure that there are in fact no other cases of SARS out there in our hospitals or in the community," Low says.

"We'll be keeping our guard up through the summer."

Low says SARS has led to a new reality in Canadian hospitals, where health care workers must remain in a constant state of vigilance while still going about their regular duties. Still, he thinks it's possible that the SARS risk will go away.

"This is not a disease of the community," Low says. "This is not an influenza that does very well in the community and stays out there all year long. This is a disease of hospitals and if you put the right precautions in place, which has been done internationally, you can break the chain of transmission and actually stop this thing."

Thirty-nine people have died of SARS in Toronto, which is the largest hotspot for the respiratory illness outside of Asia. Two outbreaks -- one in March and one in June -- forced more than 27,000 people in the city into quarantine.

With Toronto no longer on the SARS-affected list, Taiwan remains the only city in the world remaining on the WHO's roster of infected areas. Officials there are optimistic the status could change Saturday.

"The last day we had such a new case was June 15," said Chi Hsueh-yun, a spokeswoman with the Department of Health. Since then the island has gone 18 days without a new infection.

The island's total number of cases dropped by two to 674 on Thursday because two patients earlier diagnosed with SARS did not have the virus, according to Taiwan's Center for Disease Control.

With a report from The Associated Press