Luck appears to be the one distinct advantage the United States has over many other countries affected by the unfolding medical mystery of SARS, several of the nation's top doctors said yesterday.

With just 41 confirmed cases of severe acute respiratory syndrome, U.S. medical officials worry that Americans might be lulled into a false sense of security. They cautioned it would be a dangerous mistake to assume the contagious virus causing SARS will not still strike in larger numbers here.

As evidence, they pointed to two SARS hot spots -- Toronto and Hong Kong. In those cities, sophisticated medicine and strong public health systems -- on par with the expertise in major U.S. cities -- were not enough to contain the rapid spread of the disease.

"We haven't had the kind of long chains of transmission that we've seen in some other countries, but there is no reason why that couldn't happen here," Julie Gerberding, head of the Centers for Disease Control and Prevention, said on "Fox News Sunday."

"It is not under control right now," said Anthony S. Fauci, director of the infectious-disease division at the National Institutes of Health. "The important thing is to get it under control and to prevent that domino effect of expansion from one contact to another, which we've seen in a very, very serious way in Hong Kong and other Asian countries."

The new respiratory ailment first appeared in southeastern China in November. Initial confusion about the strange pneumonia, followed by widespread effort by Communist Party leaders to conceal the outbreak, gave the virus a big jump on physicians in Asia and Canada. The early cases abroad and a mid-March alert by the World Health Organization put the United States in a better position to respond.

"We were fortunate enough to be just ahead of the curve," Fauci said on ABC's "This Week."

Experts say they believe SARS is transmitted most easily through close human contact, often from sneezing or coughing. But some laboratory analysis and several cases suggest it may survive on surfaces for several hours, making it possible to spread the germ simply by touching objects.

In Hong Kong and Toronto, medical workers have been hit hard by the disease, and health systems are in danger of being overwhelmed, said Jeffrey P. Koplan, a physician at Emory University in Atlanta who just spent a week in Hong Kong.

"No matter how talented the health workers are and what resources are available, an event like this quickly taxes it to the extreme," said Koplan, who was Gerberding's predecessor at the CDC. "There is nothing that has happened in Toronto that couldn't happen anywhere in the United States."

Fauci and Koplan expressed concern about the many unknown aspects of SARS, particularly how it can be spread, how long individuals are contagious and whether someone can transmit the virus without displaying symptoms. Those uncertainties have meant that in Hong Kong virtually every patient arriving at a hospital is treated as a potential SARS case, which reduces the chances of missing SARS carriers but adds to the burdens on doctors and nurses.

"It's very difficult to sustain this level of intensity and concern and hard work for such a long period of time," Koplan said, noting that Hong Kong hospitals have been battling SARS for two months already.

Worldwide, SARS appears to kill about 6 percent of those known to be infected, Gerberding said. But with so many critically ill patients, she predicted the death toll would rise. SARS has infected at least 4,800 people and killed 293, according to the latest figures collected by the WHO. U.S. officials are monitoring more than 200 possible cases.

Because there is no known treatment or vaccine, health officials say their main tool in fighting SARS is isolating patients and quarantining others who have been exposed to the coronavirus.

"We know that our containment can work, but we do have to be vigilant about every step in the process," Gerberding said. "And that means detecting people at the earliest possible moment."

Fauci, citing an executive order recently signed by President Bush, noted that the federal government does have the power to isolate and quarantine infected patients arriving in the United States from abroad. But state health officers and legal scholars say that order does not extend to people already in the United States or in large groups, such as an apartment building, congregation or hospital.

Last week, WHO officials issued an advisory that people should postpone travel to Toronto, citing evidence that the city had inadvertently "exported" a handful of SARS cases to other countries.

Robert G. Webster, an infectious-disease specialist at St. Jude Children's Research Hospital in Memphis who accompanied Koplan to Hong Kong, said that despite the harsh economic consequences of that advisory, he supports the WHO decision.

"Show us it is under control," he said in response to complaints by Canadian leaders. "They must demonstrate they have stopped the spreading."