A virus recently linked to prostate cancer is a new suspect in chronic fatigue syndrome.

U.S. scientists tested blood from 101 patients and found two-thirds carried the virus. That does not mean the virus causes chronic fatigue syndrome, stressed the researchers in Thursday's online issue of the journal Science.

The team of scientists from the National Cancer Institute and Nevada's Whittemore Peterson Institute said it was possible the virus, named XMRV, was just "a passenger virus" that catches a ride in patients whose immune systems are weakened by the ailment.

'There is still much that we do not understand.' —John Coffin, microbiologist

Moreover, the researchers found nearly four per cent of healthy people carried the virus, too. That raises bigger questions about just what role this recently discovered virus — a relative of viruses that cause cancer in mice — may be playing in overall health.

"This suggests that several million Americans may be infected with a retrovirus of as-yet-unknown pathogenic potential," the researchers concluded.

A retrovirus is a kind of virus that permanently embeds in the body.

Various viruses have been linked to chronic fatigue syndrome over the years, only to fall by the wayside as potential culprits in the mysterious illness thought to afflict millions.

It is characterized by at least six months of severe fatigue, impaired memory and other symptoms, but there's no test for it — doctors rule out other possible causes — and no specific treatment.

The XMRV virus is related to mouse leukemia viruses. No one knows how it arose or how people become infected.

But another research team recently found the virus lurking in about a quarter of 200 prostate tumours — and in about six per cent of non-cancerous prostate samples they used for comparison.

"There is still much that we do not understand," including whether people with either disease just are more prone to infection, cautioned Tufts University microbiologist John Coffin in an accompanying editorial.

Still, "further study may reveal XMRV as a cause of more than one well-known 'old' disease."