In the end, severe acute respiratory syndrome didn’t turn out to be the viral Big One that epidemiologists have been warning about–a pandemic causing the deaths of millions. But SARS did prove that the worriers have a point: Emergent diseases can spread with great speed; the public health response
is inadequate; and the economic impact can be huge.

SARS seems to have originated in the exotic meat markets of southern China in November 2002, then festered while the Chinese government tried to suppress and conceal the problem. The disease traveled to Hong Kong in February 2003, then followed airline routes to Vietnam, Singapore and Toronto. By August, cases had been reported in more than two dozen countries in North and South America, Europe and Asia. More than 8,000 people fell ill; 774 died. The U.S. was mostly spared, with just 29 reported cases, none fatal.

At the height of the fears, Taipei subway riders were required to wear medical masks. Police guarded Toronto hospitals to ensure everyone who entered was screened for symptoms. Jittery San Franciscans and New Yorkers avoided local Chinatowns. Several airlines suspended most flights to Asia, suffering major financial losses. Perhaps the most terrifying aspect of SARS was the ease with which it spread. A Chinese doctor who stayed a single night at a Hong Kong hotel may have infected up to 16 people–many of them, experts theorize, when he sneezed while waiting for an elevator.

SARS was nowhere near the most deadly disease of 2003: Far more people died from the flu. But public health officials warn we should expect more such outbreaks, and worse ones: Continued incursions into the environment give micro-organisms the opportunity to jump from wild animals to human hosts, and air travel practically ensures that diseases go global. And as bad as SARS was, it’s easy to envision something worse–more contagious, more deadly. “I look at SARS as a dress rehearsal for something bigger,” says Stuart Cohen, an epidemiologist at the University of California, Davis. “There will always be emerging diseases. How our public health system responds will be critical.”

In a heroic effort, a network of 13 labs in 10 countries identified the SARS coronavirus barely two months after China first notified the World Health Organization of an odd pneumonia outbreak in Guangdong Province. Researchers soon sequenced the virus’s entire genome, but that knowledge didn’t lead to a vaccine, much less a cure; it only provided a slowpoke way to confirm suspected cases.

What ultimately brought SARS under control were old-fashioned methods: isolating people with symptoms (high fever, aches and a dry cough) and tracking down those they’d come in contact with. The global response was swift, experts say; still, an outbreak that infected a tiny percentage of the world’s population taxed public health systems to their limits. “Our bench science and high tech were beautiful, but when it came to control, it was a real struggle,” says Cohen. The number of health care workers infected–in some countries they accounted for more than half of those who fell ill–exposed weaknesses in the system. Hospitals must ensure that employees take basic precautions such as washing hands and properly disposing of contaminated items. They also need negative-air-pressure rooms in which to isolate patients. Barry Bloom, dean of the Harvard School of Public Health, wrote in Science that the U.S. should be training epidemiologists and strengthening lab capabilities around the world: “This investment would protect our country and every other against global epidemics (and) save millions of lives.”