The new lung infection that has triggered an international health emergency is unquestionably caused by a previously unknown virus related to germs that cause the common cold, the World Health Organization announced yesterday.

Dutch scientists have produced the final pieces of evidence needed to conclusively link the microbe, known as a coronavirus, to the disease, severe acute respiratory syndrome (SARS), scientists at the United Nations body concluded.

While researchers have been focusing for several weeks on the virus as the probable cause of SARS, the definitive connection is nonetheless a milestone in the global health crisis. It will allow scientists racing to fight the epidemic to focus exclusively on the virus. It could speed development of better tests, aid efforts to find a treatment and accelerate work to produce a vaccine.

"We now know with certainty what causes SARS," David L. Heymann, executive director of the WHO's communicable disease program, said in Geneva. "Now we can move away from methods like isolation and quarantines, and move aggressively toward modern intervention strategies, including specific treatments and eventually vaccination. With the establishment of a causative agent, we are a crucial step closer."

Separately yesterday in Beijing, WHO officials investigating the epidemic in China sharply criticized the Chinese government for withholding information from its citizens and the world. Officials said there could be as many as 200 SARS cases in Beijing alone -- five times the official tally.

Heymann, meanwhile, said that although the primary route of SARS transmission is through droplets that infected people spray out when they sneeze or cough, scientists had detected evidence of the virus in feces and urine.

This would explain why many victims experience diarrhea and other gastrointestinal symptoms, and it could mean that the virus can be transmitted through waste. That would provide an alternative explanation for how the disease spread rapidly through a Hong Kong apartment tower, an outbreak that has puzzled and alarmed researchers, he said.

Efforts to contain the disease have focused primarily on identifying and isolating sick people before they spread the virus. That appears to have been effective in some places, such as Hanoi, Singapore and Toronto. But the disease is still spreading in Hong Kong and mainland China, and sporadic new possible cases continue to be reported throughout the world.

While several tests have been developed for the virus, none is precise enough to answer key questions, such as whether people can spread the virus before and after their symptoms begin and end.

Although much more research is needed to determine where the virus originated, the microbe's genes do not appear similar to any other known coronavirus, suggesting that it has been hiding in nature in southern China, perhaps in an animal, said Masato Tashira of the National Institute of Infectious Diseases in Tokyo.

There were only two previously known human coronaviruses -- thus named because of their distinctive, crown-like shell -- and they cause about one-third of all colds. Other coronaviruses infect many different animals, sometimes causing severe respiratory or gastrointestinal diseases.

The announcement firmly associating the coronavirus to SARS was made at a meeting of scientists from 13 laboratories in 10 countries that WHO called to review the state of knowledge about the disease, one month after the agency declared an international emergency because of it. The scientists represent an unprecedented global network the WHO quickly assembled last month to fight SARS.

The conclusion came after researchers in the Netherlands produced two final pieces of evidence: Monkeys infected with the virus developed a disease identical to that seen in humans; and scientists then were able to find the virus in the animals.

"We can with all confidence say the causative agent of SARS is the coronavirus," said Albert Osterhaus of the Erasmus Medical Center in Rotterdam, who led the research.

Researchers at the University of Hong Kong were the first to isolate the virus. Scientists at the U.S. Centers for Disease Control and Prevention in Atlanta quickly confirmed that the microbe was a coronavirus. Other laboratories soon found evidence of the same virus in more patients.

But some scientists also found signs of another virus, known as a paramyxovirus, and Chinese scientists said they had evidence that another organism, called chlamydia, might be involved.

The WHO network started building the case for the coronavirus. For a virus to be conclusively linked to a disease, it must satisfy four criteria, known as Koch's postulates:

The microbe must be found in all patients with the disease; it must be isolated from a patient and grown in the laboratory; it must reproduce the disease in an animal; and it must be found in that animal.

The Dutch research satisfied the final two criteria, showing that monkeys infected with the coronavirus developed the disease, those infected with the paramyxovirus did not and those infected with both did not get sicker than if they were infected with the coronavirus alone.

WHO is calling the pathogen the SARS virus, and dedicated the discovery to Carlo Urbani, the WHO scientist who alerted the world to the nascent epidemic before dying from the disease.

The speed with which the microbe was identified, its genome deciphered and its link to the disease proved was unprecedented, illustrating the sense of urgency surrounding the epidemic. Scientists suppressed urges to withhold information for individual fame or profit, officials said.

"All of this has been overcome by the understanding that we are being faced with a public health emergency," said Klaus Stohr, who is coordinating the WHO network.

The SARS epidemic apparently began in the southern Chinese province of Guangdong in November. It spread to Hong Kong by an infected doctor and then radiated elsewhere through air travelers visiting Hong Kong.

At least 3,293 suspected cases have been reported in 23 countries, and 159 victims have died. Hong Kong and southern China have been hit hardest. Outside Asia, Toronto is the worst SARS hot spot.

U.S. health officials are investigating at least 199 cases in 34 states, including five in Virginia.

Correspondent John Pomfret contributed to this report from Beijing.