LIKE earthquakewatchers in California, doctors around the world have been waiting for the “big one”. Diseases come and diseases go, but it is thought inevitable that one of them at one time will be so quick to spread, incurable and so lethal that even the full force of modern science could not stop millions of deaths.

The bubonic plague killed 25 million people in Europe in the 14th century, the Spanish flu in 1918 killed 70 million. Aids has killed 12 million so far, but it has taken many years to spread, and — where there are sufficient resources — it has proved easier to control than first feared.

As the Sars outbreak, the first new disease of the third millennium, has spread around the world at jumbo-jet speed, as the deaths have mounted day by day, doctors have watched nervously, asking many questions, but with just one at the top of their minds: “Is Sars the big one?” Certainly, the panic is big. Just six weeks after the World Health Organisation brought the mysterious severe acute respiratory syndrome to the world’s attention with an emergency alert, it has spread to about 30 countries and more than 4,000 people, and caused some 280 deaths.

Beijing has closed all its schools and called off its May Day celebrations. Hospitals and housing blocks are under quarantine in Toronto and Hong Kong. The World Health Organisation has given warnings not to go to Toronto, Hong Kong or parts of China, airlines are screening passengers for signs of sickness and have cancelled hundreds of flights, and holidaymakers have cancelled tens of thousands of trips.

In much of the Far East, people wear masks and refuse to shake hands with strangers. Chinese restaurants worldwide have suffered a collapse in business, and in North America customers are shunning cinemas and supermarkets.

British schools have quarantined pupils in country houses and hospitals. GPs have been put on alert. As international travel has collapsed, the Organisation for Economic Co-operation and Development has given warning of global “financial turmoil”.

The story began last November in the city of Foshan in the rural Guandong province of southern China, when a microscopic virus jumped from an animal — possibly a chicken, perhaps a pig — to a human and, instead of dying, it multiplied and spread. The first officially recorded case was a 40-year-old businessman. More cases occurred in the citywith people running high temperatures, coughing and, occasionally, dying. The virus multiplies rapidly in the lungs, the body defends itself by pumping in immune-system fluid, forcing people to gasp for breath. One in twenty sufferers drown in their own body fluids.

By January, Guangzhou, the provincial capital of Guangdong, was awash with stories about the mysterious illness, which spread at frightening speed. The Chinese Government banned the media from reporting it because of fear of endangering “social stability”.

Liu Jianlun, one of Guangzhou’s top doctors, who had been treating Sars patients, went to a family wedding in Hong Kong, taking the 45- minute flight to the densely populated territory. He checked into the Metropole Hotel and entered the lobby lift coughing, unwittingly infecting 12 other people. He developed the pneumonia-like condition and, on his deathbed confessed what it was to doctors at the Kwong Wah Hospital. By March 4 he was dead, and 77 hospital staff were infected.

The guests at the Metropole Hotel, unaware of whom they had been sharing the lift with, carried on with their lives. Many turned out to be “super-spreaders”, each capable of infecting a vast number of people.

One infected guest ended up in the Prince of Wales Hospital in Hong Kong, where he infected 70 staff and a kidney patient. A few days later, the kidney patient visited his brother at the Block E apartment building in Amoy Gardens in Hong Kong, infecting about 300 people. Hong Kong authorities quarantined the whole block, banning everyone from coming and going.

Another Metropole guest, Kwan Sui-Chu, 78, flew back to her family in Toronto, infecting her son and daughter-in-law, and dying on March 5. Since then, Canada has had 330 cases and 16 deaths.

Johnny Cheng, a Chinese-American businessman staying at the Metropole Hotel, flew to Hanoi in Vietnam, where he infected his staff and 22 nurses in the local hospital.

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