Atlanta ? The mysterious and deadly flu-like illness from Asia that has stymied health officials around the world appears to be caused by a new variety of a common cold virus, U.S. health officials said Monday.

The head of the U.S. Centers for Disease Control and Prevention said a number of tests had revealed traces of a form of microbe known as a coronavirus in the tissue of people infected with the unidentified disease.

“There’s very strong evidence to support coronavirus” as the cause, said CDC director Dr. Julie Gerberding at a news conference. However, she cautioned more testing was needed before experts can be certain.

Many different viruses can cause colds, including three different varieties of the coronavirus. Until now, in fact, the cold is the only human ailment known to be caused by coronaviruses, but the CDC said the culprit in the Asian outbreak appears to be genetically different and probably represents a fourth type.

Just what that means for treating severe acute respiratory syndrome, SARS, which has killed 17 and sickened 456 worldwide — including 39 in the United States — since Feb. 1, is unclear. There aren’t any federally approved treatments for the common cold.

But there has been progress with antivirals against other respiratory viruses, particularly influenza, and some of those drugs have been effective in studies against some coronaviruses. The Defense Department will test the virus against all known antiviral drugs to see whether any work.

Researchers worry that any virus they find in patients may be an innocent bystander.

Among the bits of evidence suggesting the new coronavirus really is the cause is the discovery that victims develop an immune system response to it during the course of their illness.

The coronavirus is responsible for 10 percent to 20 percent of common colds and respiratory illnesses that sometimes lead to bronchitis and pneumonia.

Some forms of coronaviruses can be much more dangerous, causing significant lower respiratory tract illnesses among animals, said Dr. Frederick Hayden, clinical virologist for the University of Virginia Health System.

“If this is coronavirus, it certainly would be a novel one, different than how the recognized human coronaviruses behave,” Hayden said. “But, if confirmed, it’s a very important step forward in trying to … re-understand its epidemiology and to devise methods for coping for it.”

The illness first gained attention in Hong Kong, Singapore and Vietnam, where it has spread among health workers in hospitals. Health authorities say it appears to spread from close contact, primarily through nasal fluids by coughing or sneezing.