TORONTO, May 10 -- The disease investigator was anxious. A terrifying, invisible illness was spreading like a predator in the city. It was already days ahead of him, and the detective knew he had just hours to catch up with it before it killed again.

But on this day in mid-March, Mark Bartlett had few clues to work with. Public health officials in Toronto had only recently begun to suspect that the disease here might be associated with the one that was already ravaging parts of Asia and had just been given a name: severe acute respiratory syndrome, or SARS.

Bartlett and a health inspector, Henry Chong, both from Toronto Public Health, entered a suburban townhouse looking for clues to what killed one of its residents, a 78-year-old woman who had recently visited Hong Kong. They also were hunting for airline tickets or anything else that would indicate whether she had transported a deadly disease to Toronto on a jet from Hong Kong, where SARS cases were numerous, and who else might have been on that plane.

Their search and those conducted by other public health officials around the city eventually led to the containment of SARS in Toronto, not by solving the scientific mystery surrounding the disease but by tracing the people who were carrying it. Old-fashioned, gumshoe detective work -- finding people and isolating them -- was instrumental in stopping the disease’s spread.

Toronto had become the epicenter of the biggest outbreak of the virus outside Asia. Twenty-three people died in the city, more than 300 were infected -- half of them health care workers -- and 10,000 were quarantined. Before the danger was considered to have passed, public health investigators worked morning to night, interviewing hundreds of people, probing their faulty memories, backtracking, looking at diaries, using a police database, breaking into buildings -- following any lead that might reveal the path of the killer disease.

“If we didn’t contain it, it had the potential to spread,” said Colin D’Cunha, Ontario’s commissioner of public health. “One simply could not predict the consequences of that. We had the opportunity to catch it in Toronto, and if we didn’t catch it, it had the potential to spread in North America.”

In Toronto, where more than half the population was born elsewhere, public health officials thought they were prepared to deal with contagions brought from countries where infectious diseases have not been eradicated. But this microbe, whatever it was, baffled them

“We didn’t know at the time whether it was bacteria or a virus,” said Bonnie Henry, associate medical officer of health for Toronto. “We weren’t sure what the incubation period was. We weren’t sure quite how it was transmitted, whether it was droplets, whether it was contact or whether it was airborne. There was no way to tell whether someone had been exposed and was not yet ill. There is no test to tell if somebody was going to get ill. There is no way to tell if they actually have the disease. There was no treatment and no vaccine.”

When Bartlett and Chong searched the townhouse in the suburb of Scarborough on March 17, they knew that one of its residents, Sui-chu Kwan, had died on March 5, 11 days after flying home from Hong Kong. They also knew that her son, who had picked her up at the airport, had died a week later. But they didn’t know when the woman had contracted the disease or when she was most likely to have passed it to others.

“It was unclear whether the woman, who had died at home, had been ill prior to leaving Hong Kong,” Henry said. “So we were concerned about the flight.”

They swept the room with their eyes, scanning for subtle clues in much the same way that a detective dusts for fingerprints. “At that point in the chronology, we were looking for anything out of the ordinary,” Bartlett said. “We didn’t know what we were dealing with.”

While Bartlett went from room to room, Chong checked the air in the house for carbon monoxide or any other gases. They looked in the kitchen, then went up to the bedrooms.

“You never know when you are going in someone’s house what you might run into,” Bartlett said. “There were many unknowns. The main thing was: Was there going to be something there that was critical that I wouldn’t pick up on? When you are dealing with the unknown, there could be something there causing a problem, but it looks normal.”

Bartlett looked in the bedroom, under the bed and finally in the closet. There sat the luggage from the trip. The baggage tags were still attached to the handles. A passenger list was quickly secured, passengers were contacted, and none was found to have been infected. That meant that Kwan was unlikely to have passed the disease to anyone until after she had reached Toronto, which sharply narrowed the number of threads the investigators had to follow.

Bartlett’s discovery did not end the SARS crisis in Toronto. But along with clues turned up by other investigators, it helped focus the effort to contain the disease. There was still work to be done.

Odyssey of a Killer Microbe

The story of how the virus came to Toronto begins with Kwan, a 78-year-old woman who immigrated to Canada years ago.

On Feb. 11, she and her husband flew to visit their son, who lived in Hong Kong’s Kowloon district, according to Donald Low, chief microbiologist at Toronto’s Mount Sinai Hospital. The couple’s Continental Airlines vacation package included a pass to stay six nights at a hotel of their choice.

After staying at their son’s house for six days, they decided to take advantage of the hotel offer. They chose the Metropole, which promised “elegantly appointed” rooms and a stunning view of the city. They checked in on Feb. 17.

They checked out Feb. 21, the same day a professor from China’s Guangdong province checked in. The professor had been looking after people in Guangdong who were suffering from a deadly new illness, identified only as an acute respiratory syndrome. By the time he checked into the Metropole, he was already feeling quite ill. He was assigned a room on the ninth floor, the same floor as the Kwans.

The next day, the professor checked out and went to a hospital, where he subsequently died. The professor became the index case, or the first carrier of the disease, in Hong Kong.

No one knows whether Sui-chu Kwan met the professor, rode the elevator with him or simply touched the same elevator button.

“Knowing what we know about contact spread . . . they might have shared the same elevator ride,” Low said. “It might have been the professor contaminated the elevator button with the virus they might have subsequently acquired. He was responsible for nine other cases of SARS that went on about the world.”

Kwan and her husband returned to Toronto on Feb. 23. Her son picked them up at the airport and drove them to the townhouse they shared with two sons, a daughter-in-law and a 5-month-old boy. “She looked like she was just tired from the flight,” said Low, who talked to family members. But by the following day, she had started to develop what appeared to be a chest cold. She went to her family doctor, who told her she had the flu and prescribed antibiotics. But Kwan, who had diabetes and heart disease, became increasingly ill.

Wednesday, March 5: Kwan died in her sleep.

Paramedic Wayde Lansing remembers getting the call at about 6 that morning. “I was at ground zero with the very first patient,” he said later. Lansing recalled that Kwan’s husband sat next to her in her room. He remembered her two sons mourning. But he could not recall the face of the woman who would come to be known as Patient 1. He said it was a coping mechanism he used in his job. Otherwise the faces of the dead “would haunt me,” he said.

It appeared Kwan had died of cardiac arrest. People often die of heart attacks early in the morning. Lansing needed to call the coroner, but his cell phone was not working. He remembered asking to use the phone in the townhouse. It was the same phone that Kwan’s son used to call 911. It was the same son who had just given his mother mouth-to-mouth resuscitation.

By habit, Lansing used a tissue to wipe off the phone, but it was just an ordinary tissue, not the type soaked in antibacterial chemicals that promises to clean 99.9 percent of germs. “I called the coroner. I offered my condolences. I shook hands with the young gentleman without gloves at this point,” he said.

Later, after Lansing came down with SARS-like symptoms, he would recall the visit to the Kwan house as the first of six calls he made over several days and the one that might have exposed him to a microbe that no one knew anything about.

Friday, March 7: Kwan’s eldest son, Chi Kwai Tse, developed a fever, cough and chest pain. He checked into Scarborough Grace Hospital, where doctors diagnosed pneumonia.

Tse spent the night on a gurney in the emergency room, waiting for a room in the crowded hospital. A thin, cotton curtain, only about six feet away, separated him from a 76-year-old man who was suffering from heart disease. The two never met, but they would later become known as Patient 2 and Patient 8.

Tse was finally transferred to intensive care, and doctors thought he might have tuberculosis. “It is a common diagnosis in that part of the city. We see a quarter of the cases of tuberculosis in the city of Toronto,” Henry said. “We have a large immigrant population here . . . and many people come from areas where tuberculosis is really common.”

That experience proved valuable in containing SARS. So did the time that many of them had spent in Third World countries.

“I had worked in Uganda during the Ebola outbreak. I did work with TB. I had a higher comfort level,” Henry said. Ebola was incredibly deadly, causing its victims to bleed to death, and Henry was not afraid of this new illness. She tracked people down and isolated them instead of sending her staff to do the work. “The initial interviews,” she said, “I did myself until we became more comfortable.”

Saturday, March 8: Tse’s condition deteriorated. He was having trouble breathing. His oxygen levels fell. Public health inspectors began an investigation for tuberculosis, trying to locate everyone Tse had been in contact with. But as the week progressed, it became more apparent they were dealing with something else.

“It wasn’t presenting like TB,” Henry said. “And his tuberculosis test came back negative. That made us think, ‘Wow, what else is going on?’ “

Meanwhile, Patient 8 was discharged.

Sunday, March 9: Doctors asked Tse’s family to come in for X-rays -- five adults and three children. “The doctor looking after the young man realized a couple of the family members were a lot sicker today than they had been, and he again called us and said there is something else going on here,” Henry said. “And we made arrangements to have four family members who were sick assessed at the hospital” in isolated rooms.

Monday, March 10: The X-rays showed that three out of four family members had signs of pneumonia. They remained hospitalized, and the doctor treating them suspected TB and ordered them to wear masks so they wouldn’t spread it.

That day, Patient 8 returned to the hospital with a fever. “More importantly,” said Low, “his wife came with him. She sat out in the waiting room and she was unwell. Not a lot of attention was paid to her.” She infected some of the other people in the waiting room. The hospital was eventually closed to contain the virus.

Patient 8 was moved to cardiology, where he infected unknowing doctors and nurses. One of the doctors transferred him to York Central Hospital, where the virus spread still further.

Wednesday, March 12: The disease now called SARS was taking such a severe toll in Hong Kong, southern China and Vietnam that the World Health Organization issued a worldwide alert: “Until more is known about the cause of these outbreaks, WHO recommends patients with atypical pneumonia who may be related to these outbreaks be isolated.”

But doctors in Toronto still did not connect Tse to what was happening in Asia. “The pieces of the puzzle were not filling in here,” Henry said. “This man had not traveled in many years out of Canada. This was a man whose mother had died at home, at the time thought to have died of a heart attack.”

James Young, Ontario’s commissioner of public security, said Toronto had bad luck. “Unfortunately for us, it was described one day on the Internet, and the next day it showed up in the hospital. Even if we read it on the Internet and asked [Tse], ‘Had you traveled?’ his answer would have been no. It was detective work by public health that figured that out.”

The other bad luck for Toronto, Young said, was that Tse was highly contagious. “He was a super spreader.”

Thursday, March 13: Tse, Patient 2, died. By now, public health officials were beginning to suspect his death was connected to the atypical pneumonia outbreak in Asia. Interviews with the family offered the first sign that his mother had just returned from Hong Kong.

“At that time, our primary concern was people who had contact with the young man and mother before her death who might be out in the community,” Henry said.

Medical detectives fanned out across the city, carefully interviewing people who were possibly caught in the web of the spreading virus. What had they done in the previous 10 days? Who had they lunched with?

Friday, March 14: Henry met with one of the Kwans who had been hospitalized on March 9, the daughter of Patient 1, to find out who had attended her mother’s funeral.

“I spent several hours Friday afternoon with her in ICU trying to piece together what had happened over the previous 10 days with her mother and brother,” Henry said. When Henry realized they might not be able to get all the names of the people who had contacted the family and who were at the funeral, she asked the daughter whether officials could release her mother’s name to the public. That would serve to warn anyone listening to radio or watching television that they may have been in contact with a new infectious agent. The woman, designated Patient 3, agreed.

“She is a heroic woman,” Henry said.

Public health officials held a news conference asking anyone who had contact with Kwan or her son to call a hotline.

Saturday, March 15: One of the first calls was from a doctor who had treated a member of the Kwan family. The doctor, a 37-year-old woman, would be identified as Patient 7. “The family doctor was admitted,” Low said, and soon other health care workers were reporting illness.

“It was that morning when a nurse just happened to mention to me she was somebody who never gets sick. She walked by me and said, ‘I had a temperature last night when I was on duty.’ She played it down, not realizing the significance of that,” Low said.

There were several doctors who had seen members of Kwan’s family. One of them, public health detectives discovered, was on a cruise with his wife, who was also his secretary. The doctor covering his practice had no idea where he had gone and didn’t have keys to his office.

Desperate to alert the doctor, health investigators tracked down a building manager, then a part-time secretary, then the doctor’s brother. But no one could produce keys to the office.

“We were very concerned about getting in contact with patients who had been in the office when this person had been there sick,” Henry said. “So we made arrangements with the brother and the part-time secretary and the building manager to break into the building.”

Soon it was clear that the investigators had made a start -- but the crisis was not yet over. The microbe was still moving ahead of them.

It took three more weeks, 246 more cases and 20 more deaths before, on April 29, WHO decided that the danger here had passed and lifted a week-old advisory against travel to Toronto.