TORONTO — A new medical study records in stark terms the impact one undiagnosed case of SARS can have on a hospital, on a health-care system, on human lives.

When a man with an undiagnosed pneumonia went to the emergency department at Scarborough Grace Hospital on March 7, he touched off a wave of infection that struck 128 people in and around that hospital alone, according to the study, rushed to print Tuesday by the Canadian Medical Association Journal.

In the weeks that followed, 17 people from that group died.

Twenty-two per cent of nurses who worked in Emergency before Scarborough Grace realized what it was up against fell ill with SARS; 60 per cent of nurses who worked in the coronary care unit with some of those early unidentified SARS patients became infected.

Transfers of infected but unrecognized patients to other hospitals led to another 118 cases around the city.

The city's second SARS outbreak, which encompassed 119 cases, was seeded by the unfortunate events of March 7. In fact, the city's entire tragic experience with SARS can be traced to the exposure of hospital workers, patients and visitors to that one initial patient.

"I think every single one of the patients that we have (in Toronto) can be traced back eventually to the index case . . . except for four travel cases," Dr. Bonnie Henry, one of the authors of the study, said in an interview Tuesday.

It's clear evidence of why everyone involved in the fight against SARS in Toronto is desperate to ensure city hospitals remain free of the disease.

"It's just a good reason why you don't want this disease in your hospital unrecognized, because this is the kind of attack rate you can see," said Dr. Donald Low, chief microbiologist at Mount Sinai Hospital and one of the leading figures in Toronto's fight against SARS.

"It's a bad, bad, bad disease in the hospital."

It's been known since March that Scarborough Grace was the epicentre of Toronto's devastating SARS outbreak. But this study - by disease investigators from Health Canada, Mount Sinai and Toronto Public Health - details for the first time just how hard that hospital, its staff and patients and their families were hit.

"It's really very neat the way they've traced all the transmission," said Dr. Andrew Simor, head of microbiology at Sunnybrook and Women's College Health Sciences Centre and another key SARS expert.

"And it again just confirms how even a single case of a disease which is potentially very communicable under certain circumstances can spread so easily to such a large number of individuals in a very short time - and certainly does emphasize the importance of the unrecognized or the undetected case."

Traced almost all of the transmission, to be more precise. For the first time, disease investigators admitted they don't know how eight cases from the Scarborough Grace cluster became infected. Those eight were patients or workers on a unit apart from the wards where SARS was known to be spreading.

"We haven't yet figured out who they were exposed to. They were exposed in the hospital, but we haven't been able to track down who touched who sort of thing," Henry said.

"We have a number of theories that we're working on, particularly that there was an unrecognized ill person, either a staff person or a patient, on that unit for a period of time. But we're still working on that."

All the other cases trace back to a 43-year-old man, the son of a woman named Kwan Sui-chi. Kwan, 78, became infected with SARS at a hotel in Kowloon, near Hong Kong on Feb. 21 and brought the disease back to Canada with her.

She died at home on March 5.

But her son, who had helped care for her, became gravely ill. He showed up in the emergency department at Scarborough Grace on March 7, five days before the World Health Organization alerted the world to the fact that a new atypical pneumonia it dubbed severe acute respiratory syndrome was sweeping in an alarming fashion through China, Hong Kong and other parts of Southeast Asia.

When it became apparent that family members visiting him were also ill, hospital officials placed them in respiratory isolation on the assumption they had tuberculosis.

By the time the WHO made its frantic announcement, Scarborough Grace was riddled with SARS. But it was only in the weeks that followed that public health investigators were able to trace all the tentacles of the spread.

Analysis of the Scarborough Grace outbreak revealed some interesting facts about the disease's progression there.

Seven children were infected; all were household contacts of workers, patients or visitors who caught the disease at the hospital. They ranged in age from 21 months to 17 years. None died and none were sick enough to require a stay in Intensive Care.

Younger SARS patients survived the disease in most cases. The fatality rate for those under age 60 was 2.9 per cent. But for those over 60, the disease often followed a different path; nearly 54 per cent of those patients died.

Among 10 older patients who were already in hospital - and who therefore had existing health conditions - the death rate was a startling 90 per cent.

"That part of it is quite frightening," said Henry, an associate medical officer of health with Toronto Public Health and one of the key - albeit often behind the scenes - figures in the SARS containment battle.