It's a frightening medical mystery. A new disease that is sweeping across borders and has already caused tremendous panic in Asia.

This pneumonia-like illness, dubbed SARS for severe acute respiratory syndrome, has infected more than 2,300 people worldwide and resulted in 81 deaths, says the World Health Organization.

Along with China's Guangdong province, Hong Kong has been particularly hard hit, with hundreds of cases leading the government to quarantine many of those who may have been exposed.

American students there were sent home this week when their school was shut down because of SARS. "The whole campus is evacuated, all the students are gone and there's still cases popping up in different dorms," says Steven Rubbinaccio, a student at Chinese University of Hong Kong.

"I am probably going to go to either a doctor or a hospital right now to be checked out," adds student Matt Foran.

Even physicians in far-away Boston are sensing growing anxiety. Says Dr. Michael Shannon, an emergency room pediatrician at Boston Children's Hospital: "We've had at least three visits I'm aware of where families are specifically asking, 'Could my child have SARS?' "

This newest global health threat emerged just months ago in a remote province in China. How could it spread so widely and with such stunning speed?

ABCNEWS' Dr. Tim Johnson explains experts believe one doctor treating patients in China caught SARS, then traveled to Hong Kong. There, he infected 12 other guests of the Metropole Hotel where he was staying. That set off a deadly global chain reaction.

A major outbreak followed in Toronto because another Metropole hotel guest, a Chinese-born woman and husband were visiting relatives in Hong Kong before returning to Toronto. She fell sick and within days was dead. Before long, other family members started to get the tell-tale SARS symptoms: body aches, fever, a cough and difficulty breathing.

Doctor Detects One of the First U.S. Cases

In the United States, the Centers for Disease Control and Prevention has had reports of 115 suspected SARS cases in 27 states.

Dr. David Pitrolo, an emergency room physician at Chesapeake General Hospital in Chesapeake, Va. believes that in mid-March he treated one of the country's first cases.

On the morning of March 15, Pitrolo saw a story on CNN about the mystery illness that was infecting hundreds in Asia and prompting health organizations to issue travel advisories. Later that same day when he went to work, one of the first patients he saw was a woman with flu-like symptoms who had just returned from Asia.

The patient was isolated, the CDC contacted, and the case confirmed on March 19. "She had the travel history, which was our tip-off," explains the observant physician.

Pitrolo worries about what will happen if the disease spreads and the travel history is no longer a determining factor. "The symptoms are pretty general," he adds. "It looks like the flu and there are no lab tests to help you [make a SARS diagnosis]."

Identifying the Unknown

While experts can't say yet what is causing it the spread of SARS, the latest research points to a virus called the coronavirus, so named because it has a crown-like appearance under magnification.

Dr. Kenneth McIntosh, an infectious disease specialist at Children's Hospital in Boston, literally wrote the book on the coronavirus. He says normally it's harmless, causing nothing worse than the common cold.

But he suspects this new virus is dangerously different — that it started out in China in an animal then jumped to humans, making it far more deadly.

"I am frankly quite concerned," says McIntosh. "I share the CDC's sense that this is a potentially very important event."

McIntosh also believes all of the infection control and prevention actions being taken to stem the virus' spread may not be effective. "It's going to be very difficult to completely stop the spread of this virus. My bet is it's going to spread."

Diagnosis and Shot of Prevention

Experts are reportedly working on developing a test that can diagnose SARS infection, a step that others say first involves sequencing the virus' DNA.

"I think that a widely usable diagnostic [test] is still a ways off," says Dr. Elaine Tuomanen, chair of infectious diseases at St. Jude's Children's Research Hospital in Memphis, Tenn.

Tuomanen says once the exact sequence of the virus is known and researchers discover what parts of the virus are making people sick, a vaccine could conceivably be developed. In fact, scientists at St. Jude's were recently able to manufacture a vaccine against a lethal strain of influenza that caused the World Health Organization to issue an alert in February of this year.

The vaccine for the influenza strain H5N1 was developed in four weeks, and Tuomanen says that the same technology could be utilized to protect people against SARS.

"This one particular technology is extremely rapid and allows you to tailor make a vaccine," she adds. "This is a very big deal because it makes everyone feel safer and it can be done."

ABCNEWS' Susan Wagner and Melinda T. Willis contributed to this report.