Despite an unparalleled global counterattack, severe acute respiratory syndrome (SARS) has likely become a permanent threat that will plague humanity indefinitely, infectious disease experts say.

A scientific consensus appears to have emerged that the window for stamping out the dangerous new lung infection has probably closed because the SARS virus has become entrenched in many places.

"It's definitely here to stay," said Michael T. Osterholm, director of the University of Minnesota's Center for Infectious Disease Research and Policy.

What remains far from clear is whether the epidemic will continue to expand and become a greater public global health emergency, or settle into the landscape of human diseases that cause sporadic outbreaks of varying size year after year.

"The question is, 'Is this going to be a public health nuisance, a public health problem or a public health crisis?' " Osterholm said. "That's the million-dollar question."

In the long term, SARS is likely to affect some countries more severely than others, experts said. China probably will remain hardest hit, while countries where the disease has been rare so far, such as the United States, may continue to see few cases.

"It would be hard for me to see how it could be eliminated from places like China or Hong Kong at this point," said James Hughes of the National Center for Infectious Diseases in Atlanta. "I think it would be prudent to say it's here to stay. The jury's still out in terms of whether it becomes established here."

Other specialists said that it's highly unlikely that SARS will disappear from any place where people have become infected.

"It could gradually escalate and escalate and escalate. It could undergo a rapid burst of cases. Or it could stay at this level and peter out because of public health measures," said Anthony S. Fauci of the National Institute of Allergy and Infectious Diseases. "It could also peter out now, and, like we see in many cases with influenza, it could come back next season with a vengeance. Any one of those is plausible."

However the epidemic unfolds, SARS has demonstrated both the power of modern medicine to fight a new microbial menace and the limits of even today's science when nature spawns a novel pathogen.

"It's been a very impressive story about how quickly various technologies have been brought to bear," Steven S. Morse of Columbia University said. "But yet the disease continues to spread."

To contain any disease transmitted through the respiratory system is difficult. So it might have been impossible under any circumstances to contain SARS in China. But any chance of squelching the nascent epidemic was lost because the Chinese government concealed the disease for so many months, experts said.

Since SARS emerged in southern China in November and began spreading internationally in late February, the illness has extended to almost every corner of the planet. As of Saturday, the World Health Organization said more than 3,500 cases have been reported in at least 25 nations. Hundreds of people have been hospitalized, thousands have been quarantined and at least 182 people have died. In the United States, officials are investigating more than 200 cases in at least 34 states, including five in Virginia.

SARS circumnavigated the globe rapidly because it is the first dangerous new disease in decades that can be transmitted person-to-person -- most commonly through droplets sprayed out by a sneeze or a cough. In addition, intercontinental air travel enabled infected people to transport the virus internationally in record time.

The WHO, already on high alert because of earlier reports that a dangerous flu bug had resurfaced, responded with unprecedented urgency. It issued global alerts, mobilized the world's best scientists and marshaled intensive containment efforts. Because of heightened fears about bioterrorism, countries such as the United States were ideally poised to rapidly identify and isolate cases, which is key to stemming a budding epidemic.

The response snuffed out some early hot spots, identified the viral cause in record time and focused intense pressure on the Chinese government to be more forthcoming about the outbreak. Nevertheless, SARS continued spreading to one country after another, and the number of suspected cases reached hundreds and then thousands within weeks. Once an infectious disease has spread widely, eradicating it becomes almost impossible.

"We can't control tuberculosis in the world. And that is clearly a disease that doesn't look to be as easily transmissible as this," Osterholm said.

Researchers have taken early steps to develop a SARS vaccine, but that weapon is years away at best -- and even widespread inoculation may not be able to stamp out SARS.

Despite concerted efforts against many infections, smallpox remains the only communicable disease to have been wiped out. Wide-scale vaccinations were able to eradicate smallpox because it infected only humans. One of the many unanswered questions about SARS is whether it can hide in animals.

"If SARS is coming from another natural source, such as from another species, then it's quite possible that it could be reintroduced periodically," Morse said. But even if there is no animal reservoir, SARS could continue spreading.

"Once you've established a disease in the human population, you don't really need an ever-present animal reservoir," Fauci said. "There's enough spread among humans that it is more likely to establish itself as a smoldering reservoir in humans."

No other diseases are exactly comparable to SARS, but it could develop a seasonal pattern like flu.

One of the key determinants is the unresolved question of exactly how the SARS virus spreads. While the primary mode is believed to be close personal contact, the virus did sweep rapidly through an apartment tower and hotel in Hong Kong in ways that remain unclear.

"There are enough of these outliers to tell us that we don't know what the full potential is," Fauci said. "There is something unusual and beyond the scope of face-to-face contact."

During the weekend, the WHO said SARS appears to have been more severe in the Hong Kong apartment building outbreak than elsewhere and sickened more younger, otherwise healthy people. That raises fears the virus may have mutated into an even more dangerous form, the WHO said.

Another key unknown is whether the disease can be spread when people have no symptoms.

"If that's the case, and there are thousands and thousands of people who have gotten infected and not gotten sick, then it amplifies greater the possibility of persistent human infection," Fauci said.

That's why officials have made developing an accurate diagnostic test the highest priority.

"This is a disease that is now probably endemic in China. It's definitely an epidemic in Hong Kong, as it is in Toronto," said Donald Low of Mount Sinai Hospital in Toronto. "Patients will carry it to other countries and introduce it into new countries. Some of those countries will be able to respond adequately and other countries won't have the resources or expertise. It's bleak. It really is bleak."

Three people wear masks as they leave Prince of Wales Hospital in Hong Kong, which has been hit hard by severe acute respiratory syndrome.