The Food and Drug Administration yesterday recommended that anyone who has had severe acute respiratory syndrome (SARS) or is at risk for the lung infection be barred from donating blood, a move designed to protect the blood supply from the new illness.

Although there is no direct evidence that the SARS virus has spread through blood transfusions, the agency said it was taking the step as a precaution until more is learned about the disease.

"At this time, there are no known cases of SARS transmission via blood products. However, detection of the genes of the possible causative virus in blood has been reported in a patient with SARS," the FDA said.

SARS is believed to spread primarily through droplets sprayed out when an infected person sneezes or coughs, but the virus has been detected in every body fluid that has been tested, including blood.

Regulations require that blood donors be in good health, which would ensure that anyone with an active case of SARS would not donate.

In new guidelines issued to blood collection agencies, the FDA said donors should also be questioned to determine whether they may have had the disease, been treated for it, recently traveled to parts of Asia where the disease is most common -- China, Hanoi and Singapore -- or had close contact with a probable or suspected case or someone who traveled to those places.

Anyone who has had the disease should not donate for 28 days after any symptoms and treatment have ended. Anyone who has traveled to affected areas should not donate for two weeks after his or her return, the FDA said.

In addition, anyone who has donated blood and had either been exposed to the disease in the previous two weeks or had the disease within the previous 28 days should report that to authorities.

"Donors should also be encouraged to report SARS illness or treatment that occurs within 14 days after donation," the agency said. Blood from those donors "will be quarantined and indefinitely kept out of the general blood supply," the FDA said.

The measures are not expected to significantly affect donation, because the number of people who would be turned away should be relatively small, no more than 0.4 percent of donors, the FDA said.

Also yesterday, the Centers for Disease Control and Prevention in Atlanta announced a change in the way it classifies the cases it is investigating to comply with the World Health Organization definition.

Of the 208 suspected cases in 34 states, 35 were consistent with WHO's definition of a "probable" case. Of those, all but two had traveled to Asia. The two are a health care worker and a family member who apparently were infected by a traveler.