Thank Your Lucky SARS; Health officials may have dealt this virus a death blow.

Byline: Jaime Cunningham and Stefan Theil, With Sarah Schafer, Jen Lin-Liu, Alexandra A. Seno and Fred Guterl

Back in December, as northern weather was getting cold and runny noses were once again becoming the norm, health officials braced for a return of SARS. It didn't come until last week--and not the way it was supposed to. Health experts had assumed that the virus was so well ensconced in the natural world that we were unlikely to avoid a repetition of last year, when the disease jumped from animals to humans. But last week's outbreak--the biggest so far this year--was entirely man-made. Two workers at Beijing's National Institute of Virology apparently caught the bug from lab samples. A graduate student infected her mother (who died) and a nurse at the hospital where she was being treated. The nurse, in turn, infected five others. All this happened just before the May Chinese holidays, as 90 million potential disease carriers were preparing to travel.

Nobody expects this outbreak to approach the horror of last year, when SARS infected more than 8,000 people from Guangdong to Toronto and killed 774. This time Chinese health officials tracked down the source of the infection, quarantined the lab and got the word out--quickly--to hospitals. The difficult questions are: Why did we get off so easy? And how did world experts get their prognostication so wrong?

The short answer is that nobody really knows. Scientists still don't understand SARS. They've yet to come up with a vaccine, antiviral treatments or even a reliable and cheap diagnostic test. Given all the uncertainty, health experts last autumn erred on the side of caution. "I don't think there was any scientific basis to say there was going to be another outbreak," says Robert Webster, a virus ...