On 23 February 2003, a Continental Airlines flight from Hong Kong landed in Toronto carrying a 78-year-old grandmother and her husband.
Toronto was clearing up from a winter storm that had dumped 20cm (8in) of snow. But in sweeping up after one storm it cleared the way for another. The aircraft that touched down on the runway brought with it a lethal organism that, two months later, would in effect shut down Canada's commercial capital.
When the World Health Organisation (WHO) took the unprecedented action of advising against all "unnecessary" travel to Toronto yesterday, it was signalling that Sars is on the threshold of becoming a global epidemic. Toronto, one of the world's most orderly cities, was up in arms at being singled out for such draconian treatment.
But for the medical establishment, battling to contain the virus, the fact that this city of four million is less than a day's drives from half the population of the United States was a more compelling factor.
Although the grandmother, Kwan Sui-Chu, had been infected with Sars and had brought it home to Toronto, at that point no one had even heard of the disease. In Hong Kong she and her husband had stayed on the ninth floor of the Metropole Hotel at the same time as a Chinese professor of respiratory medicine, Liu Jian Lun, who is now known to be the case that triggered the global epidemic.
Professor Liu had been treating patients with a strange, pneumonia-like illness in the Guangdong province of southern China. A single sneeze in the Metropole's lift lobby may have been enough to infect seven other people who subsequently spread the illness round the world. One of them was Kwan Sui-Chu.
Two days after her return to Toronto, where she lived in a town house in the suburb of Scarborough with her husband and grown-up sons, Mrs Kwan developed a high fever. She already had heart problems and diabetes and her family did their best to keep her comfortable.
On Friday, six days after arriving home, she went to a local doctor, who prescribed antibiotics. By this time her 43-year-old son, Tse Chi Kwai, who shared his parents' home with his wife and five-month-old son, was also starting to sicken.
Sometime later Mrs Kwan slipped into a coma and on 5 March she died in her own bed. The doctor called to the family home in Scarborough wrote "heart attack" on the death certificate. The virus that had actually killed her was meanwhile on the rampage.
Her son's condition was becoming worse, with fever, sweating and a dry cough – now known to be the classic symptoms of Sars. He visited his doctor who X-rayed him, prescribed antibiotics and sent him home. His wife had also fallen ill, and Mr Tse took her to see her own doctor, Ada Ying Tak Lo, a GP who ran two surgeries in Scarborough. She was to become the next victim in the chain of infection.
The next day Mr Tse was so weak that his sister took him to Scarborough Grace hospital where he lay in casualty for 12 hours. During that time it is estimated 200 other patients passed through the unit.
Next to him on a trolley was Joe Pollack, 76, who needed treatment for an irregular heartbeat. Ten days later Mr Pollack and his wife were dead. By this time the WHO had issued its first global alert on Sars, on 12 March, but Canada was not included. Doctors treating Mr Tse in the Scarborough Grace hospital thought at first he was suffering from tuberculosis. They ordered X-rays on him and seven family members, which showed lung infections in three adults. They learnt too that his mother had recently died, that she had been in Hong Kong and could have got ill there.
Doctors decided that TB was a public health threat and they should take precautions. Mr Tse was moved into isolation and his family were told to wear masks and stay at home. They informed the public health department but although they were aware of cases of unexplained respiratory illness in Asia, no one made the connection.
So far the disease had been confined to one family. But now Dr Ada Ying Tak, who had treated Mr Tse's wife, developed a severe headache and fever. She booked a week off work. She was to be the first indication the authorities had that the disease, whatever it was, had spread beyond the Scarborough grandmother's family and was now at large in the community.
The WHO's global alert on 12 March said Sars had spread beyond national borders to infect people in China, Hong Kong and Vietnam. In Hong Kong dozens of doctors and nurses had been infected. The WHO recommended patients be isolated and anything that came into contact with them be sterilised. But in Toronto, suspicions were still not aroused.
The next day Mr Tse died in the Scarborough Grace hospital. The same afternoon, the hospital's infection control officer called Dr Alison McGreer, an infection disease specialist at Mount Sinai hospital in Toronto, for advice. Dr McGreer, who had seen the email alerts from the WHO, made the connection. At last, nearly three weeks after the Sars virus had arrived, doctors were alerted that they were dealing with a lethal new disease.
All members of Mrs Kwan's family, many of whom were by now sick , were admitted to isolation wards at different hospitals. The Center for Disease Control in Atlanta, Georgia, was alerted and a press conference arranged. On Friday, 14 March, the world learnt that Sars had arrived in Canada.
After the press conference at Mount Sinai, Dr McGreer and her colleagues went up to her office and jotted down on a white board the names of the family and how each may have become infected. At that stage they were confident the infection was contained to one family, two of whose members had travelled to Hong Kong.
Their confidence was shortlived. The following morning, Dr Tak Lo, the GP who had treated Mrs Kwan's daughter-in-law, called Mount Sinai to say she was sick. It was a major setback.
Toronto's public health department swung into action. Doctors set out to track down all the passengers on the Continental flight that had brought the Kwan couple home. Infection control procedures were stepped up and preparations for quarantine made.
But it was too late. The virus was already on the loose. Ten thousand people in Canada have now been quarantined as the medical authorities struggle to contain the new disease.
Yesterday, as the WHO conferred international pariah status on Toronto, it said the outbreak in the area had "continued to grow in magnitude outside the initial risk groups of hospital workers , families, visitors and other close person to person contacts."
The questions on every Canadian's lips now is: How far will it go?
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