In just a few short weeks, SARS swept out of nowhere to become a grave new threat to world health. And now the virus is mutating rapidly -- into a showcase for unproven products and technology.

With the severe acute respiratory syndrome creating social unrest in Asia and anxiety everywhere, scores of companies, many of them small ventures that have spent years laboring in obscure fields of science, are rushing forward with possible solutions. Governments around the world are suddenly confronted with a need to police these claims and crack down on charlatans, even while they prod larger companies to embrace SARS research.

Claims of SARS cures are spiraling out of control on the Internet, with companies offering "natural" remedies alleged to ward off the disease. SARS is so new that none of these claims could possibly be backed by scientific evidence; the few treatments that doctors have managed to test on SARS patients, such as the antiviral drug ribavirin, have been failures.

Government agencies in the United States and Canada said yesterday that they have launched an enforcement campaign to stop the most egregious Internet claims, with warning letters to dozens of companies. "Bogus products from questionable Web sites do no good, and can actually make matters worse by providing a false sense of protection," said Mark McClellan, the U.S. food and drug commissioner.

The sudden rush of offerings from small companies comes as the big drug firms have been reluctant to undertake SARS research. Drugmakers still don't know if SARS will fizzle or turn into a global pandemic that kills millions. The government has been cajoling them to get started on SARS research, but the only thing the big companies are likely to produce in the near term is a reliable test for the SARS virus.

Some of the recently introduced SARS products are scientifically credible undertakings, even if their commercial potential remains unproven.

Among the most promising is a new "gene chip" from Affymetrix Inc. of Santa Clara, Calif., a company that has built a successful business offering similar products. Affymetrix chips are small glass wafers that contain microscopic genetic detectors. The company said this week that it had created a chip that may allow doctors to figure out whether some SARS strains are more likely to make people sick -- and to zero in on potential weak points common to all strains.

The Affymetrix chip illustrates the speed of modern biology. No Western scientist had even heard of the SARS virus as recently as February, and now an Affymetrix production line in Sacramento is cranking out test lots of a device capable of detecting every molecular variation among the 29,000 units of information that make up the SARS genetic map. But this is also a relatively new application of a technology used mostly for other purposes. Affymetrix failed commercially several years ago with a similar chip for analyzing the AIDS virus, as researchers chose other analytical techniques.

People at the company said they felt a responsibility to get a product out quickly.

"We have done zero business analysis on the SARS" chip, said Greg Yap, senior marketing director for DNA analysis products at Affymetrix. "We do try to be rigorous about our business in most cases, but in this case we felt that the time urgency and the health needs were enough to allow us to skip all the analysis."

Some of the technologies that companies are pushing have established uses, but their application to SARS is speculative. Asian governments have deployed infrared detectors at airports to assess body temperatures as people get off airplanes, in hopes of finding passengers who are feverish. Such detectors have proven useful as quality-control devices in manufacturing and other fields, but there is little evidence that they can reliably distinguish a person carrying a virus from a person who isn't.

Many people crossing international borders are likely to do so after they are infected but before they begin to show symptoms. Others may take steps to avoid detection. Some passengers are taking aspirin during flights to reduce their temperature.

None of that stopped John M. Brenna, president of Computerized Thermal Imaging Inc. of Portland, Ore., from pitching his company's infrared detectors at a hearing in Congress the other day, urging lawmakers to push a SARS thermal-imaging network for the nation's borders. The company has sold 11 thermal systems in Asia. As evidence of the medical potential of his company's devices, Brenna told the lawmakers that Computerized Thermal Imaging was "working with the FDA to obtain approval" of infrared detectors as a diagnostic device for breast lumps.

Computerized Thermal Imaging does have FDA approval for an infrared detector to diagnose pain, but prospects for the breast system that Brenna touted in front of Congress are highly uncertain. An FDA advisory committee voted 4 to 3 in December against the company's device, and the FDA subsequently rejected it, saying the company's tests were inadequate. The company is appealing.

Brenna also did not mention other woes that have beset the company. Its stock price has collapsed, it is low on cash, and federal investigators have subpoenaed records from the company and its chairman, Iran-contra scandal figure Richard V. Secord, in connection with large stock sales he made just before the FDA panel voted against his company.

Brenna said yesterday that he didn't feel those details about the company were relevant in his congressional testimony. He said the company would support any government effort to collect better data on how well infrared detectors can stop SARS. "It's a first-defense screening mechanism," Brenna said. "That's all it is at this point."

Public-health advocates are generally wary of technological fixes. Instead, they have called on Congress to reverse decades of low investment in systems and personnel to monitor public health. "I'm skeptical, in the real world, that this kind of high-tech system is our best use of money," said Shelley A. Hearne, executive director of Trust for America's Health, an advocacy group.

Some of the new SARS offerings are drawing unsought government attention. The FDA, the Federal Trade Commission and a government watchdog agency in Canada said yesterday they were cracking down on Web sites selling untested herbal products as SARS cures. The operators of several Web sites changed the language of their advertisements after receiving FDA or FTC warning letters.

Not every Web site operator is spooked, though.

Consider a new "SARS formula" said to contain, among other ingredients, "methanolic extracts of two berry bushes." It was being offered yesterday on several Web sites, including www.cureforsars.net, controlled by one Leonard Horowitz. Calling from Hawaii, he described himself as a doctor and the developer of the product. He said it included ingredients that had been shown in the test tube to work against a cow virus related to SARS, but he acknowledged that it had not been directly tested against SARS. He plans to offer various formulations, with the deluxe version selling for $60 for a two-ounce bottle.

Though Horowitz's Web sites describe the product as a "cure for SARS" and an "effective SARS treatment," he said he had received no recent FDA or FTC warning, and it wouldn't bother him if he did. "The FDA has absolutely no place or business issuing to us or any other herbal manufacturer any type of citation," he said. "They can warn us all they like. It's not going to affect anything. Literally what they would have to do is come and jail us."

It is true, he said, that people need to be careful what they buy over the Internet.

"I'm sure there are some charlatans out there," Horowitz said. "It's very sad."

A coronavirus is projected on a screen yesterday at the Genome Institute of Singapore. SARS is caused by a coronavirus.