U.S. health authorities say a previously unknown virus might be the cause of the Asian pneumonia that has killed 17 people worldwide and infected more than 400 others as of Monday. It is the third virus to become a suspect in the case.

Officials at the U.S. government's Centers for Disease Control, the CDC, say they have detected a a new kind of virus in patients infected with the respiratory disease called SARS.

CDC director Julie Gerberding says this virus is the leading suspect as the cause of the illness because several different types of laboratory tests have found it in the blood, lungs and other tissues from nine patients around the world. "It isn't enough to just find virus in one patient and one tissue," says Ms. Gerberding. "What we're looking at here is the convergence of a variety of different methodologies, all of which are pointing us in this direction right now."

The virus in question is a coronavirus, two strains of which cause one-third of common colds.

Coronaviruses are not usually deadly, but Dr. Gerberding says an examination of the genetic structure of this one indicates it is a new type. Other viruses have also been implicated in SARS. German and Hong Kong laboratories extracted a paramyxovirus from the noses of patients, while Canadian health laboratories found a metapneumovirus in others.

The CDC chief says her agency has not detected these viruses in its specimens, but is keeping an open mind that the coronavirus might even work in concert with the others. "There are a lot of other potential explanations for what we are finding here and we are exercising caution and not being dogmatic that we have the answer here. The other laboratories have said the same thing," says Ms. Gerberding. "The challenge here is that we have a very nonspecific illness, and we're dealing with families of virus that are ubiquitous [common]. Finding them in tissues is not the same thing as saying they are causing the disease."

The Centers for Disease Control will continue to examine tissue samples from as many SARS patients as it can to determine if the coronavirus is, indeed, the cause. There are no drugs for coronaviruses, but whatever the cause of this disease, the agency says control is best carried out by isolating patients, as is the practice for tuberculosis.

SARS is believed to have been brought to Hong Kong by a Chinese physician, and it has spread from there. Dr. Gerberding says all cases have been transmitted by close personal contact and public health authorities are not seeing a more general spread through communities.