during SARS, a select few people inexplicably ended up infecting a dozen, two dozen or more people, turning a disease that might otherwise never have been spotted into a four-month worldwide panic.

SARS went global thanks to a superspreader — a Chinese doctor who infected more than a dozen people at a Hong Kong hotel in late February. One of those people brought the virus to Canada.

In Singapore, one SARS patient infected 62 people. In Toronto, which had several superspreaders, one early case infected 44 others.

In fact, an elderly couple who contracted the virus on the night SARS made its first appearance in a Toronto hospital were both superspreaders.

The woman, who had taken her husband to hospital for a heart problem, brought him back a few days later when he began to suffer from the symptoms that would come to be recognized as SARS. Later, people who traced the spread of the virus through Toronto hospitals would see that she infected three admission clerks, a security guard, five visitors, three nurses and one housekeeper — all within a 2 1/2 hour span.

"She wasn't that sick, actually. I don't even know if she had a fever," says Dr. Donald Low, the Toronto microbiologist who helped lead the city's SARS response.

"But clearly she was excreting a lot of virus ... which then ping-ponged into a massive number of cases throughout the city."

It's not clear why some people became superspreaders during SARS.

True, in some cases the amplified transmission seemed to relate more to the circumstances than actual patient. For instance, it became apparent that intubating a patient — putting him or her on a breathing machine — could be a superspreader event if health-care workers weren't wearing respirators fitted over their noses and mouths and goggles to shield the mucus membranes around their eyes.