While public health officials leading the fight against SARS warned only that the battle has reached a critical stage, the doctors who advise them were ready to spell out the threat in more graphic and frightening terms.

The disease has travelled so far from the original cluster of infection - one expert estimates the city is on the fifth generation of cases - that the risk is mounting daily that people will go undiagnosed and will spread SARS in the community, sparking what are called sporadic cases that experts can't trace.

"That's exactly the concern," said Dr. Andrew Simor, head of microbiology at Sunnybrook and Women's College Hospital and a key member of the provincial containment team.

"We're not at that point today and hopefully we will not be, but that is the threat. That is the risk."

And a very real one at that, said Simor, who answered this way when asked if the Toronto outbreak can be brought under control:

"I think it is still possible but I am less optimistic today than I was a week ago."

Already, officials have admitted that several people who were eventually diagnosed with SARS were turned away from hospitals when they sought help because they didn't fit the profile used to make the difficult distinction between an emerging case of SARS and a garden-variety respiratory ailment.

Health-care professionals had been looking for people who had recently returned from Southeast Asia, people who had contact with a SARS patient or people who had been at one of two hospitals where uncontained outbreaks occurred. Later, people who had attended an April 3 wake for a man who died of SARS were added to the equation.

With word earlier this week that at least 30 members of a Catholic church group had become infected - and that several were turned away from hospitals because they didn't fit the SARS profile - those trying to contain the spread of the disease realized the profile is no longer a tool hospitals can rely on to identify incoming SARS patients.

The experts insist there is no evidence yet of sporadic cases in Toronto, but admit the city may be poised on the knife's edge.

"If we're extraordinarily lucky and can continue to mobilize forces in Ontario and can over the next, literally, days add the resources that public health needs to maintain the fight in Ontario, we may still have a chance to control it," said Dr. Allison McGeer, a leading infectious disease expert who is herself in hospital recovering from SARS.

"But it's going to take another infusion of an astounding amount of effort to do that."

She believes the small army that has been working 20-hour days for the past five weeks needs a huge wave of new recruits.

"Tripling? Maybe. Quadrupling? In my view, we're talking about calling in every public health resource that we can muster to move far enough ahead of this to stop it," said McGeer, who continues to work from her sick bed via phone and Internet links.

One only need look to Hong Kong to get a mental picture of what an out-of-control SARS outbreak looks like. Governments around the world have urged their citizens not to travel to Hong Kong and tourism has plummeted. Shopping malls are empty. Hospitals are paralysed because so many health-care workers are sick. The case count increases, in double-digit fashion, daily. And the death toll has reached 61.

The number of probable and suspect cases in Ontario rose to 249 today, from 244 Tuesday. On the bright side, a total of 91 SARS patients have been discharged from Ontario hospitals.

Canadian numbers topped 300 for the first time, with 303 probable and suspect cases reported, though to date the Toronto area is the only jurisdiction with a serious and spreading SARS outbreak.

"If we start to see true community spread of SARS in Toronto, the initial implications of that are going to be a major hit to tourism," McGeer predicted. "The movies (production crews) won't come. The major conventions will pull out.

"People are very, very frightened of this and it's going to get worse. And you can imagine situations in which people will not only not want to do business with people from Toronto, but they won't want to accept shipments of widgets from people in Toronto."

At a SARS briefing today, provincial officials took a slightly less ominous approach, while still trying to drive home the message that people need to take public health instructions seriously. The coming Easter weekend, with its many religious services and family gatherings, poses real challenges for efforts to contain SARS, they admitted.

"We're at a point where the critical thing that we have to do is get the co-operation of the community, even though it's a longer period of time and they're growing tired of doing this," said Dr. James Young, Ontario's commissioner of public security.

"We're at a weekend where traditionally there is a lot of activity. People are meeting. They're socializing. So that means we've got a whole lot of risk factors coming together at once and we've got a message that isn't very popular for any of us.

"We're asking a lot of people. But we're saying to them that we wouldn't be asking it if we didn't think this was the right message and a critical time."

The message is, by most measures, an extraordinary one, as Young himself readily admitted.

Anyone in Toronto who feels unwell - with headaches or muscle aches, malaise, a cough - is urged to stay home and isolate themselves from their family until they feel better. If they develop a fever, they are urged to call the province's Telehealth line and ask for instructions.

"We're asking for a level of people staying home that is quite unprecedented but very important," Young said.

Young even admitted the province is looking at whether it can use electronic tracking devices to enforce quarantine orders issued against people who've been told to isolate themselves but who refuse to comply.

The containment team is also asking employers to be lenient when employees call in sick, saying it's better to lose one employee for a few days than to have a whole shop, office or factory forced into quarantine for 10 days.

McGeer suggested employers who don't normally offer sick leave benefits to workers should do it for the duration of the outbreak. The initial expense may be peanuts in comparison to the cost of what would happen if a person with SARS showed up for work sick because he or she couldn't afford to stay home, she said.

"If we invest in it now, if we say, for the next two months: `If public health puts you on quarantine, I will pay you your hours,' we'll get it back," said McGeer.

"And if we don't do that, we stand to lose the city."

Also today, a WHO-led network of laboratories from around the world announced that a coronavirus which had been identified as the leading suspect was indeed the causative agent of what the world now knows as severe acute respiratory syndrome, or SARS.

Monkeys infected with the new coronavirus developed the same symptoms seen in the human form, they reported.

"The conclusion today, the people in the network agreed, (is) that the coronavirus alone is capable of causing the atypical symptoms," said Dr Albert Osterhaus, director of virology at Erasmus Medical Center in Rotterdam, Netherlands.

Erasmus completed the work to definitely prove that the new coronavirus causes SARS.

"The pace of SARS research has been astounding," added Dr. David Heymann, executive director of the WHO's communicable diseases programs. "Because of an extraordinary collaboration among laboratories from countries around the world, we now know with certainty what causes SARS."

But the head of Canada's national microbiology laboratory, which is one of the 13 in the international network, remains skeptical that the coronavirus is the sole cause of SARS.

"There's a spectrum of conviction out there," Dr. Frank Plummer said of the scientific world's acceptance of the coronavirus theory. ``There's something we don't understand."

Plummer said his lab continues to find the virus in only about 50 per cent of specimens taken from SARS patients and has found the virus in samples taken from a number of people who have had no symptoms of the disease and no known contact with a SARS case.