But doubt was growing among independent researchers as numerous other studies failed to find any connection between the purported infection and human illness.
Tuesday, the journal Science took the unusual step of declaring the XMRV link "seriously in question" -- as it published research that concluded the earlier connection almost certainly was the result of laboratory contamination.
Sophisticated genetic tracing from the National Cancer Institute found the XMRV virus itself arose from the combination of two other mouse viruses during some experiments about a decade ago that involved growing human prostate tumors in the animals. The virus' genetic fingerprint so closely matches what was later found in samples taken from patients, that it's extremely unlikely the XMRV could have come from another source than contamination in laboratories, the researchers concluded.
In a separate study, yet another team of researchers tested blood from the same chronic fatigue patients used to make that first 2009 link with XMRV. This new testing, which avoided using lab products derived from mice, found no evidence XMRV, further supporting the lab-contamination explanation.
In fact, substances in human blood are able to kill the mouse-related virus, said lead researcher Dr. Jay Levy of the University of California, San Francisco.
The National Institutes of Health already had begun still other studies to settle the issue.
But Levy argued it's time to move on, saying there's evidence that chronic fatigue involves an immune disorder: "Let's use the money to find the real culprit."
Researchers at Nevada's Whittemore Peterson Institute, who first reported a possible XMRV link, didn't immediately comment Tuesday.
Various viruses have been linked to chronic fatigue over the years, only to be ruled out as potential culprits. Chronic fatigue 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.