Encouraged by successes in Vietnam and elsewhere, public health experts say that quarantines, border checks and other aggressive measures to contain SARS are working and may prevent it from becoming a persistent scourge.
"It sure as hell can be contained," said Dick Thompson, spokesman for the World Health Organization, which is leading the global effort. "Look at Hanoi, where it's been done without a lot of high-tech equipment and with basic outbreak control techniques."
While the campaign against severe acute respiratory syndrome has been unprecedented in its scope, experts caution that it would be premature to predict victory. They are closely watching the virus' steady advance in China, and point out that the origins and behavior of the virus are still not well understood.
"This is a very contagious disease with very bad consequences," said Dr. Richard Chaisson, an infectious disease specialist at the Johns Hopkins School of Medicine. "If it's snuffed out now, will it disappear back into oblivion or continually reappear? Not knowing where it came from and how it behaves makes this very hard to predict."
Still, the past week brought a collective, if cautious, sigh of relief from public health experts who a few weeks ago were pondering whether SARS would spiral out of control and become a pandemic.
On Monday, the World Health Organization declared that the disease had been contained in Vietnam and was waning in Singapore, Hong Kong and Canada. Effective yesterday, the United Nations agency lifted an advisory against travel to Toronto, the only North American city to face a serious outbreak.
Scientists are optimistic that reliable tests will soon be available - possibly in a few weeks - that will enable doctors to rapidly diagnose or rule out the disease.
"A diagnostic test would be tremendously beneficial," said Allan Brandt, a medical historian at Harvard Medical School. "It would allow us to restrict contacts in appropriate rather than generalized way." Now, it's hard to tell if a person has a common flu or is in the early stages of SARS.
Hopes for a diagnostic test were buoyed by the successes of three laboratories that independently deciphered the genetic sequence of the coronavirus thought to cause the disease. That should also help speed development of a vaccine.
Dr. John Bartlett, who heads the infectious diseases division at the Hopkins School of Medicine, said he was astonished by the rapid pace of discovery.
"With Legionnaires' disease, it took months to find the bug, and developing the antibody test just plodded along," Bartlett said, referring to the respiratory illness that killed 34 people in Philadelphia in 1976. "This thing is moving along like fire."
Bartlett and others fear SARS cases will continue to climb in China. Constant vigilance will be required to keep the illness from spilling once more outside the country, where it first appeared in November and then spread to 27 other nations through infected travelers.
Dr. Megan Murray, an assistant professor of epidemiology at the Harvard School of Public Health, said it's difficult to interrupt transmission of an illness that spreads through a respiratory route because all it takes to make someone sick is close proximity. But she believes that, in some cases, it can be done, though maybe not in China.
"Infectious diseases spread exponentially, and once they've gotten this far they become incredibly difficult to contain," she said.
Murray wonders whether the success controlling the outbreak in Vietnam was really a matter of good public health work - or good luck. The virus may never have made its way into the general community in the first place, limiting the likelihood of its spread. "To some degree, it's just chance events," she said.
With SARS, public health officials are trying to accomplish something they haven't quite done before. Health authorities point to campaigns that brought fearsome diseases such as Ebola and avian flu under control, but neither had spread as widely as SARS when the alarms were first rung. And it is hard to find a precedent for containing a lethal and highly contagious respiratory illness.
Chaisson said the best analogy is avian flu, a deadly virus that spread from chickens to humans and killed six people in Hong Kong in 1997. Fearing a replay of the 1918 flu pandemic that claimed more lives than did World War I, Hong Kong officials, in cooperation with the WHO, ordered the slaughter of the country's 1.4 million chickens. People contracted the disease through direct contact with chickens.
"They nipped it in the bud," Chaisson said. "We're hoping that this can be replayed with SARS, and right now the only setting where that isn't happening is China."