The Mystery of the SARS Virus: How Is It Spread?

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One superspreader may have been a 64-year-old doctor in China who traveled to Hong Kong in late February. Before he died in March, he is believed to have infected eight people on his floor at Hong Kong's Metropole Hotel.

"It's very likely this virus is usually spread by droplet transmission, but from time to time it may be embedded in a "superspreader" who may also spread the virus through airborne transmissions," said Dr. William Schaffner, chairman of the department of preventative medicine at Vanderbilt University, in Nashville, Tennessee.

One theory suggested by medical investigators is that infected people may carry the disease without suffering extreme symptoms. Healthy carriers of a disease are not unusual, the most famous of whom was Mary Mallon. Popularly known as Typhoid Mary, she spread typhoid fever in the United States without suffering symptoms herself.

A Brand New Virus

Tests have linked SARS to a family of viruses known as the coronaviruses (though scientists are still awaiting confirmation). Until now, coronaviruses were believed to produce only minor illnesses in people, like colds, diarrhea and other intestinal disorders. In cats, dogs, chickens, pigs, and cattle, coronaviruses can cause severe and often fatal illnesses.

But SARS appears to be a brand new virus, separate from the known coronaviruses. How it has become so deadly is still a mystery. One possibility is that the virus jumped from an animal. Such leaps have happened before. The hendra virus spread from horses to people in Australia, while the nipah virus went from pigs to humans in Malaysia. However, neither virus then spread from person to person.

Another possibility is that the human coronavirus acquired genes from another, more virulent virus. Coronaviruses are able to capture stray bits of genetic material from related viruses and weave them into their own genomes, a feat biologists call recombination. Such a recombination creates new viruses, and in theory could turn a benign microbe into a biological time bomb.

Experts say SARS could grow less virulent as it reproduces inside the human body, or it may grow worse. "Until there is DNA sequencing of this virus, it's impossible to determine its origin," said Harrison.

A Wake-up Call

The long-term course of SARS is impossible to predict, experts say. While some dangerous new infections have burned themselves out, others, like HIV, have become global disasters. There are signs that disease rates are dropping in China. But they may be on the rise in Hong Kong and elsewhere.

"This [is] a wake-up call to the United States and to other countries around the world regarding the challenges that emerging infectious diseases can pose," warned James Hughes, director of the National Center for Infectious Diseases and the Centers for Disease Control and Prevention in Atlanta, Georgia.

Some experts suggest that if this outbreak dies down, the virus could pop up again with no warning or it might follow a seasonal pattern, like the flu.

"The world of infections are in constant flux and turbulence," said Schaffner. "The only constant is change. We should expect from time to time for new viruses to emerge and new epidemics to occur."

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