In 2002, an unusual form of pneumonia, later known as Severe Acute Respiratory Syndrome (SARS), appeared in southern China. The virus quickly spread around the globe, infecting more than 8,000 people. But three years later, health experts say the world got off relatively easily and the next new disease may be far worse. Doctors hope new advances in the study and treatment of SARS could help protect Asia from other, more deadly outbreaks.
The streets of Hong Kong have never been busier. In the crowded Wanchai business district, hundreds, even thousands of people are headed in all directions.
But two years ago, during the height of the SARS epidemic, it was an altogether different story.
The sidewalks were half empty, tourists stayed away, children were kept indoors and pedestrians wore medical facemasks, as a panicked city coped with a new and potentially deadly disease SARS.
Severe Acute Respiratory Syndrome appeared in southern China late 2002, apparently spreading to humans from wild animals. In less than a year, it killed nearly 800 people, and infected more than 8,000 in 29 countries.
The bulk of the cases, however, were in China and Hong Kong, and almost all cases in other countries involved someone who had traveled to southern China before becoming ill.
Some victims, in fact, caught the disease by sitting near an infected passenger on plane flights. Alarmed at how rapidly modern transportation spread the disease, the World Health Organization (WHO) warned travelers to avoid China, Hong Kong, and other countries with significant outbreaks.
Roy Wadia works with the World Health Organization in Beijing. He says that SARS was a wake up call for doctors around the world.
"SARS basically showed the world that what happens in one country can really affect the entire planet," he explained. "If a disease emerges in one country, it's just a matter of time before it may be exported around the world."
The disease has largely faded away, although doctors do not know exactly why. A handful of cases have been identified since 2003 but the threat, at least for now, has abated.
SARS was frightening in part because it was new, no one knew what caused it or how to treat it, and in part because it spread quickly, forcing communities to quarantine anyone who had contact with a patient. It was unusual in that it made almost all its victims terribly ill, requiring lengthy hospitalization.
But in SARS' wake, doctors say, critical lessons have been learned; lessons they say could help prevent a far more deadly epidemic in the future.
Researchers can now describe exactly how the SARS virus operates and are using the discovery to develop treatments for other emerging diseases.
In August's Nature Magazine, scientists in Vienna report the virus apparently interferes with an enzyme that helps regulate blood pressure.
Blood vessels in a victim's lungs break down and the lungs begin to flood, killing about one in 10 patients.
The discovery raises hopes that treatment targeting the vulnerable enzymes also could treat a range of other diseases.
At Hong Kong University, researcher John Nicholls says the new findings could help fight emerging threats like the H5N1 avian flu virus. That virus has infected birds in several countries and killed more than 60 people in the past two years.
"When we did some research on the bird flu, we found there were similar lung changes between SARS and H5N1," he noted. "So from our point of view, the findings which were published are actually quite important, not necessarily for SARS, but really for should there be this global outbreak of H5N1."
SARS also underscored the importance of regional cooperation in fighting epidemics.
When the disease originally appeared, Chinese authorities denied reports of the outbreak and then refused to cooperate with international health workers.
As a result, scientists say it took much longer than necessary to isolate the cause and identify treatments. It took considerable international pressure before China acknowledged the problem. However, experts say that since then, Chinese officials have worked more closely with international researchers.
Today, Dr. Nicholls says there is stronger regional cooperation and health care facilities in Hong Kong and mainland China are better prepared to treat new cases.
"I think the [Hong Kong] government has now worked out better protocols and there's less of the immediate situation we were in in early 2003 where the number of infected cases was going up daily, daily, and it was just, basically, let's try anything out of sheer pressure from not knowing what was going on," explained Dr. Nicholls.
Chinese doctors are also working on a SARS vaccine. The national government plans to invest nearly a billion dollars in its health infrastructure and has restructured its internal reporting system.
However, many critics fear some lessons have still not penetrated China's traditionally secretive government.
Last month, Chinese authorities banned journalists from visiting areas hit by a mysterious pig-borne disease.
At least 200 human cases of the disease have been reported, and more than 35 people have died.