A killer microbe slipped into the city last month, seemingly as benign as the common cold. The affliction arrived unexpectedly, as in a parable: A man goes to the hospital with a racing heart. He's placed in a hospital room with another man who has a hacking cough. They share the same, potentially lethal air.

The man with the racing heart gets better, goes home, only to return two days later, sicker. His lungs fill with fluid. He dies.

The family of the man, who has not been identified, grieves. At the funeral home, there was sympathy, but there also was fear. The funeral director told family members to drive themselves to the burial. No one wanted to be exposed to the mystery disease that killed their loved one. Public health officials advised them that the burial should be private. Even in death, they warned, do not get too close to the body. The mourners covered their faces at a lonely graveside service, dressed in black and weeping into white surgical masks.

They did not know it yet, but two of the mourners also were infected.

That anonymous death, Canadian officials said today, was one of six in Toronto among 144 suspected and probable cases in Canada of the disease christened SARS, for severe acute respiratory syndrome. Officials say thousands of Canadians are under quarantine.

The World Health Organization reports that at least 1,804 people worldwide have been infected by SARS and at least 62 have died.

Fifteen countries have reported cases of SARS. In the United States, the Centers for Disease Control and Prevention report 72 cases in 27 states, including three in Virginia. So far there have been no U.S. fatalities.

Yet the statistics are from a disease still largely unknown. Even its name is based on a diagnosis reached by the process of elimination. "SARS is a definition of exclusion. One rules out pneumonia and causes from any known infectious agents," says Colin D'Cunha, Ontario's commissioner of public health. Moreover, says D'Cunha, "There are still no confirmed cases."

Health officials here have been dividing victims into three categories: probable, suspected and under investigation. Symptoms include fever, difficulty breathing, a cough, a sore throat and muscle aches.

"This illness sometimes strikes with a sudden onset," says Sheela Basrur, Toronto's public health chief.

Scientists are looking at two possible modes of transmission: close contact with an infected person or airborne. "The worst-case scenario is if it is airborne transmission," says Don Low, chief microbiologist at Mount Sinai Hospital in Toronto.

Many patients, he says, are stable or getting better. A few have been released, but no one is sure they are cured. Nobody is sure what they had in the first place, and the doctors in charge of controlling this new disease can't tell whether they are still contagious.

"They have been cured symptomatically. The fever went away," Low says. "This is a study we have to follow."

Ontario officials have been trying desperately to contain a further outbreak, ordering a health emergency and isolation for anyone who may have been exposed. They convey a conflicted message: urgency, but with assurances that the risk to the general public is low, that it is safe to go to movie houses, schools and grocery stores.

"At the end of the day, life goes on," says Basrur. "We cannot live our lives by staying at home. There is no way our community can function. All of us have to work together to control this."

So they encourage the public to wash their hands and they encourage those who may have been exposed to quarantine themselves for the good of the many, which may be impossible in a society where the good of the few reigns. That is why doctors are threatening to use their powers to isolate people as "guests" of a hospital wing until their 10 days of incubation period have ended. But 10 days is a guess, because nobody knows when it begins and when it ends. SARS is gray right now. It has neither alpha nor omega.

"This is a disease that has never existed before and we don't know a lot about it," says James Young, Ontario's commissioner of public safety. "We can't say it is under control. We can say we are taking more vigorous measures."

SARS came to Toronto, officials say, when Sui-chu Kwan, an elderly woman, returned home to the suburb of Scarborough after a trip to Hong Kong. Kwan was among seven people who visited the ninth floor of the Metropole Hotel in Hong Kong between Feb. 12 and March 2. This is the floor where a local man, believed to be the "index case" for the outbreak at Hong Kong's Prince of Wales Hospital, visited a hotel guest who was a doctor from mainland China. The doctor had fallen sick a week before he stayed at the hotel. According to Health Canada, the national department of health, that contact "is considered to be the original source of the infection" in Hong Kong.

Sui-chu Kwan died at home on March 5, and nobody knew why. Eight days later her son, Chi Kwai Tse, 44, died at Scarborough Grace hospital. But other family members already were infected. The family has now become known as "the cluster" by health officials. She died. Her son died. Her daughter-in-law fell ill. Another son and another daughter got sick but recovered.

"Those patients were in hospital," Low says, "and those patients were cared for without appropriate precautions, and the reason was nobody at the time realized what the problem was. This was just another case of community-acquired pneumonia. Nobody realized it was a highly infectious agent that could be transmitted as easily as it could."

The disease jumped from the family cluster to the health care workers who cared for them. Public health officials hope those two clusters now are contained. That is what they say when they give their reports at daily news briefings, trying to reassure an increasingly concerned Canadian public. The epicenter of the disease is the hospital. Officials hope it hasn't jumped the ring beyond the epicenter. But they don't know. They are trying desperately to track everybody who entered Scarborough Grace after March 16.

On March 17, there was a report of four cases in the hospital and two under investigation. Almost all of the cases of SARS in Canada -- 124 suspected and probable cases -- are in Toronto, including two suspected cases at the Hospital for Sick Children.

As of Monday, Health Canada reported two probable and 11 suspected cases in British Columbia, five suspected cases in Alberta and one suspected case each in Saskatchewan and New Brunswick.

That last is a middle school principal who just returned home to New Brunswick from a trip to China. According to reporters, the woman attended one day of school before she fell ill. The school has been disinfected, and the parents in New Brunswick worry.

As the numbers grow, fears are on the rise in Ontario. Workers in every hospital in the greater Toronto area and in Simcoe County, north of Toronto, have been ordered to wear masks, gloves and protective eyewear to prevent transmission. Thousands of hospital people have been ordered into quarantine. Emergency rooms have been closed. All elective surgeries have been canceled. Airport workers have been asked to screen all passengers leaving Toronto on international flights.

Residents have gone out to buy surgical masks. One school was shut down after three children came down with a fever, even though public health officials told them the children most likely were not suffering from SARS. Driver's license examiners have refused to do road tests for new drivers. Hot lines are clogged.

People are getting irritated with SARS. After all, life goes on.

Some are violating isolation orders, saying they cannot afford to stay home. Rahana Aziz, who was a patient at Scarborough Grace hospital before the warning went out, refuses to stay home because she has to work. If there was a concern when she visited the hospital, she wonders, why did no one cancel her appointment or give her a mask?

At the Toronto Chinese United Church, the Rev. Karl Lam says attendance is lower than usual. For those who came, he preached a sermon to ease the fear: "Wilt thou be made whole." It is the story of Jesus healing a diseased man who came forward with nothing but faith. "We are all scared about SARS," he told his congregation. "But don't worry because God will be with us."

As doctors, nurses and paramedics -- the people on the front lines -- get sick, people wonder who will care for the ill. The Nurses Association has reported there are not enough N95 filtration masks for nurses still working. "These masks are essential gear in an event like this to stop the further spread of infection," association president Barb Wahl said in a statement. "And yet our nurses say some facilities are rationing them."

In Scarborough, a 45-minute drive outside the city, there is more suspicion. The Chinese community has begun to stay indoors. Whole malls are empty. The people who usually attend tai chi workouts aren't coming. The popular Ruby's restaurant is nearly empty after someone sent out a malicious chain e-mail saying the chef had SARS.

"The chef in the kitchen at Ruby's does not have SARS," says Frankie Lee, the restaurant marketing manager. He has gone on television, on the radio, has placed ads in local Chinese papers. "Nobody in our staff is infected. There is no such case. There is nothing wrong with Ruby's restaurant," Lee says. He thinks the e-mail rumor could have come from a jealous competitor trying to cut him down.

At this point, the motive doesn't matter as he looks at a nearly empty room that can seat 600 people. Tonight, there are 21. This is the economics of an epidemic. Lee has called the police. The police are searching for an invisible culprit on the Internet.

At Scarborough Grace, the epicenter of the disease, a guard wearing a mask watches the door. The gates of the parking lot swing open, but nobody is going in and nobody is leaving. Signs warn visitors that there is something lethal in the air.

Ontario hospitals are trying to project both composure and urgency.Paramedic Mike Larson is among the quarantined in SARS outbreak.In Toronto, hospitals put warning signs on the doors, above, while airport employees are on disinfection duty, left.At Scarborough Grace hospital, where Canada's SARS outbreak originated, security guards take extra precautions.