Canada has become the first country to ban people with chronic-fatigue syndrome from donating blood.
The precautionary move is a result of Health Canada's concern over a virus known as XMRV, which has only potentially been linked to chronic fatigue.
XMRV closely resembles the AIDS virus, prompting fears it can be similarly transmitted through blood transfusions. The virus has also been linked to prostate cancer.
Canada's blood supply is managed nationally through Canadian Blood Services, which has erred on the side of caution since the tainted blood scandal in the 1970s and 1980s that saw thousands of blood recipients contract HIV and hepatitis C.
Dr. Dana Devine, the vice-president of medical and research affairs for Canadian Blood Services, said in the wake of the new policy, "we have no evidence that the blood supply is less safe than it was.
"While we don't know yet whether or not this virus causes this disease, we thought it was an appropriate precautionary measure to take until the science sorts itself out," said Devine.
"No one has ever found if this virus has caused any disease in humans."
An estimated 340,000 Canadians have chronic fatigue syndrome, but Devine said only a tiny fraction of those with disease would have been healthy enough to donate in the first place.
Devine said she expects the new guidelines to take effect by the end of the month.