Viruses have nothing to do with chronic fatigue syndrome (CFS), despite earlier evidence of a link, a study has shown.
The new findings deepen the mystery surrounding the cause of debilitating condition, also known as myalgic encephalomyelitis (ME).
Scientists dismissed previous claims that two viruses, known as XMRV and pMLV, may underlie CFS.
"The bottom line is we found no evidence of infection with XMRV and pMLV," said Dr Ian Lipkin, a member of the research team from Columbia University in New York City. "These results refute any correlation between these agents and disease."
CFS can strike out of the blue - causing severe tiredness as well as muscle weakness, aches and pains, memory loss, and poor sleep.
Once dismissed as a purely psychological problem, experts now agree that it is a serious physical illness.
In 2009 and 2010, separate studies found the two viruses in the blood of CFS patients, raising hopes of identifying an easily treatable cause of the condition. But since then, other investigators have been unable to confirm the results.
"We went ahead and set up a study to test this thing once and for all and determine whether we could find footprints of these viruses in people with chronic fatigue syndrome or in healthy controls," said Dr Lipkin.
The research, published in the American Society for Microbiology's online journal mBio, involved comparing 147 diagnosed patients with CFS and 146 healthy individuals.
Blood samples were tested for genes specific to XMRV and pMLV, as had been done before. But great care was taken to ensure no contamination of the testing chemicals which may have led to false results in the earlier studies.
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XMRV and pMLV are viruses commonly found in mice, but there has never been a definite case of them infecting humans.
The tests found no evidence of the two viruses in the blood of either CFS patients or healthy participants.
Dr Lipkin said: "We've tested the XMRV/pMLV hypothesis and found it wanting. We are not abandoning the science. The controversy brought a new focus that will drive efforts to understand CFS/ME and lead to improvements in diagnosis, prevention and treatment of this syndrome."
Patients with CFS have benefited from cognitive behaviour therapy (CBT), which teaches people to change the way they think, and structured exercise programmes.
Often patients spontaneously recover after years with the condition, or learn to live with their symptoms.