Today was the second day in a row that China had no new cases of SARS, a new and untreatable pneumonia caused by a corona virus, down from a high of more than 100 new cases a day just a month ago.

Scientists think that a corona virus, of the family that causes many common colds, causes SARS, and that SARS, like colds, may be seasonal.

The international experts confirmed the official assertion that new cases in Guangdong were declining and praised the province for its aggressive efforts to contain the disease, which is believed to be caused by a form of corona virus.

"This is a very sensitive period for us in Hong Kong because this is a period where there may be some of these corona viruses that may possibly come to Hong Kong," Dr. Yeoh warned.

They said that the corona virus, which causes the common cold, remains the likeliest cause of SARS.

Finding an Experienced Health-Care Provider What is corona virus?

And, with the new corona virus silently breeding in their bodies, the nine guests at the three-star Metropole Hotel dispersed on airplanes like bees carrying a deadly pollen, seeding SARS locally and to far corners of the world.

American officials have warned that SARS, believed by doctors to be caused by a corona virus, may prove to be a seasonal disease that returns in the winter.

But Roy Wadia, a W.H.O. spokesman in Beijing, said the same samples also tested positive for another corona virus that causes the common cold, and that further tests were needed.

It is still possible, experts say, that tests are picking up another form of corona virus that is similar but not identical to SARS.