TORONTO - It struck terror in the hearts of the global medical and public health communities, rocked economies, and killed almost 800 people.
But it's been nearly three years since the disease known as severe acute respiratory syndrome has been detected in a human being.
So what happened to SARS?
As more and more time goes by, scientists are coming to believe the strain of the SARS coronavirus which triggered the dreadful new disease may have been permanently extinguished when the last of chain of SARS cases was interrupted.
"I think it's gone, period," says Dr. Donald Low, the Toronto-based SARS expert who from as early as the summer of 2003 predicted SARS was a viral one-hit wonder.
"It could well be that this particular virus is extinct now," agrees Dr. Ab Osterhaus, head of virology at the Erasmus Medical Centre in Rotterdam. It was Osterhaus's laboratory that first proved the newly identified coronavirus caused SARS.
Low's theory brings to mind an image of a lightning strike that ignites a fire. Put out the fire and the threat is gone.
He - and most of the scientific world - believes a precursor coronavirus, one that probably lives in bats, found its way into civet cats in China.
Occasionally in the fall of 2002 these viruses managed to infect people, probably people who handled or ate civet cats. But these animal viruses weren't well adapted to infecting humans and they didn't spread from their initial victim to others.
But then something changed - the lightning strike. The constantly mutating virus acquired an ability to pass from person to person and the disease that came to be known as SARS took off, spreading around the globe in a viral maelstrom.
In the early months of 2003, the world's top scientists were deeply concerned this new disease would gain a foothold in humankind, becoming a permanent member of the rogues gallery of pathogens that circulate among people, like polio, measles or influenza.
But then, under the leadership of the World Health Organization, all centres fighting outbreaks managed to break the chain of transmission of the virus in June of 2003. As Low sees it, this nightmare of a virus then hit a dead end and died.
That's not to say other SARS-like viruses don't exist in nature. Many have been found in bats and in civet cats. Studies of blood samples of people who work in China's animal markets suggest some of these viruses have occasionally infected people, but didn't trigger SARS-like illness.
Experts believe these SARS-like coronaviruses are cousins, not identical twins of the outbreak strain.
Low, who is chief microbiologist at Toronto's Mount Sinai Hospital, bolsters his argument by pointing to the dozen or so cases of SARS that were recorded in the winter and spring of 2003-2004 - the last known cases of SARS.
Three unrelated cases occurred in the community in China. Then, in April of 2004, two lab workers at the National Institute of Virology in Beijing were accidentally infected. One touched off a chain of seven other infections, including one death.
The lab-related cases, infected with virus from the 2003 outbreak, provoked the type of disease the world saw during the SARS crisis. The other three cases were mild, suffering the cold-like symptoms people get from human coronaviruses.
That could well illustrate the difference between the viruses now in the wild and the outbreak strain, Low says. And he believes it's highly unlikely any of those SARS-like viruses could randomly mutate to become as dangerous as the outbreak strain.
Others hope he's right - but think it's too soon to rule out the possibility entirely.
"I think the actual outbreak virus is, as far as we know, not around other than in laboratories," says Dr. Larry Anderson, director of viral diseases at the U.S. Centers for Disease Control in Atlanta and a key player in that institution's SARS response.
"Now the virus that . . . was first introduced into humans and then was able to adapt to become the outbreak virus, that may still be circulating."
Anderson says the lightning analogy makes sense. "But the lightning can strike again."
"I think I would agree (with Low) that it's not likely. But I'm not quite sure what likely is," he says.
Osterhaus is also cautious, believing it's still possible that one of the SARS-like viruses in the wild might acquire the ability to jump from human to human and cause serious disease.
"I would not be surprised if yet another (corona)virus from a similar reservoir would knock on the door again," he said from Rotterdam.
