The virus is known as xenotropic murine leukemia virus-related virus, or XMRV. Some studies have found that people with chronic fatigue syndrome are more likely to carry the virus. But it remains far from clear whether the virus causes the disease.

Nevertheless, the Red Cross decided to bar people with the syndrome from donating “in the interest of patient and donor safety,” according to an announcement from the organization.

“There is currently insufficient data to conclude that XMRV is transmitted through blood transfusion. However, the National Heart, Lung and Blood Institute [NHLBI] Task Force is conducting research to determine the frequency of the virus in the donor population, whether it is transfusion-transmitted, and whether recipients become infected and develop the disease,” it said.

Between 1 million and 4 million Americans are believed to suffer from the syndrome, which causes prolonged and severe fatigue, body aches and other symptoms. The cause has long been a mystery. Over the years, many viruses have been linked to the syndrome, only to end up being a dead end.

A task force that reviews blood safety for an organization known as the AABB recommended in June that blood-collecting organizations “actively discourage potential donors who have ever been diagnosed by a physician with chronic fatigue syndrome . . . or myalgic encephalomyelitis [ME], from donating blood or blood components. In addition, any donor with symptoms of CFS would be deferred if, on the day of donation, they respond negatively to the question, ‘Are you feeling well today?’ ” the agency said.

The recommendation came after new research strengthened the possible connection between the virus and the syndrome.

“The Red Cross has implemented the AABB recommendations and has gone further to implement indefinite deferral for donors who reveal a history of a medical diagnosis of CFS,” the statement said.

The Food and Drug Administration, which regulates blood safety, had no immediate comment on the decision. The agency is convening a panel of outside experts to review the issue this month.

K. Kimberly McCleary, president and CEO of The CFIDS Association of America, said the organization endorsed the move, both for the safety of blood transfusion recipients and syndrome patients, whose condition can make blood donation difficult.

“This is just added protection all the way around,” she said.