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Health officials closed a second Toronto-area hospital to new patients and asked hundreds of its employees to quarantine themselves as a deadly flu-like illness continued to spread worldwide.

Anyone who has worked at York Central Hospital since March 16 has been asked to stay in their homes for 10 days to try to contain the outbreak of severe acute respiratory syndrome, hospital president Frank Lussing said Saturday. The hospital has 1,800 employees.

The disease has killed three people in Canada and caused authorities to advise thousands of people in Toronto, the largest city, to quarantine themselves at home. Worldwide, the disease has killed at least 54 people and sickened nearly 1,500.

U.S. health officials said Saturday that none of the antiviral drugs and other treatment they have tested are effective against SARS.

On Saturday, the disease killed Dr. Carlo Urbani, the World Health Organization doctor who was the first to identify the latest outbreak when it appeared in Vietnam.

In Hong Kong, Christians and Taoists held special services on Sunday to pray for an end to the epidemic. Thirteen people have died in Hong Kong and dozens of new cases are diagnosed there daily.

Singapore said it would station nurses at its airport to examine all travelers arriving from infected areas.

All hospital workers in the Toronto area have been ordered to wear masks, gloves, gowns and protective eyewear to prevent infection from the highly contagious virus.

Two hospitals that treated victims of the illness have been shut to the public, and the government said it would begin screening travelers flying out of the city for symptoms.

Despite dozens of suspected cases, authorities said none of their current patients were in critical condition.

"The two patients I was concerned about last week have been improving over the last few days (and) didn't require ventilatory support," said Dr. Don Low, the chief microbiologist at Mount Sinai Hospital.

Most cases involve health care workers at Scarborough Grace Hospital and York Central Hospital who became infected while treating initial victims, all of whom had traveled in Asia or had close contact with other victims.

"What we're seeing in the last week is mostly health care workers that are young, healthy people; and we have to realize if we start to see individuals and families that are older, that they are more susceptible to a worse outcome," Low said.

Dr. Hanif Kassam, a medical officer for the York Region in north Toronto, said the number of probable cases there exceeded 20.

"It is very likely that we will continue to see an increase in the number of cases, and that whatever steps are taken, there will be a certain time period before we start seeing a reduction," he said.

Most of Canada's SARS cases are in Toronto, which has large Asian immigrant communities. Other cases have been reported in Vancouver, British Columbia, and Winnipeg, Manitoba.

Canada's government has strengthened a travel advisory regarding SARS, saying people going abroad should avoid even passing through Asian countries where the illness has been detected.

The United States urged citizens to postpone travel to mainland China, Hong Kong, Singapore or Hanoi, Vietnam.