Some sufferers of ME have such severe symptoms they are confined to bed
US scientists say they have made a potential breakthrough in understanding what causes the condition known as chronic fatigue syndrome (CFS) or ME.
Their research in the journal, Science, suggests that a single retrovirus known as XMRV does play a role in ME.
They found the virus in 67% of ME patients compared to under 4% of the general population.
But experts cautioned that the study did not conclusively prove a link between XMRV and ME.
ME is a debilitating condition that affects an estimated 17 million people worldwide.
The discovery raises hopes of new treatments for the condition.
Retroviruses are known to cause neurological symptoms, cancer and immunological deficiencies.
The Whittemore Peterson Institute in Nevada, said they had extracted the DNA from XMRV in the blood of 68 out of 101 patients with the condition.
Causes chronic fatigue and muscle pain
Impairs immune system
Does not improve with sleep
More women than men suffer from it
Condition controversial in 1980's when some medical authorities doubted whether it was a genuine physical illness
Cell culture experiments revealed that the patient-derived XMRV was infectious.
The researchers said these findings raise the possibility that XMRV may be a contributing factor to ME.
XMRV is also known to have a role in some prostate cancers.
Dr Judy Mikovits, who led the study, said: "It's a blood borne pathogen that we contract through body fluids and blood transmission.
"The symptoms of ME - chronic fatigue, immune deficiencies, chronic infections - are what we see with retroviruses.
"This discovery could be a major step in the discovery of vital treatment options for millions of patients."
Tony Britton, of the ME Association said: "This is fascinating work - but it doesn't conclusively prove a link between the XMRV virus and chronic fatigue syndrome or ME.
"Many people with ME/CFS say their illness started after a viral infection, and a number of enteroviruses and herpes viruses have also been implicated in the past.
"ME/CFS is an immensely complex illness, with many possible causes and there are up to 240,000 sufferers in the UK desperate to get better."
Invest in ME are enormously encouraged by the current research which shows a potential new cause for this devastating neurological illness. More importantly it promises a diagnostic test is within reach.
A spokesman for Invest in ME said: "This is a huge step achieved in such a short time and will bring hope to all people with ME and their families.
"We now call on the UK government, the Chief Medical Officer and the Medical Research Council to support our view that only a research strategy based on adequately funded and coordinated biomedical research into ME will succeed in creating treatments and eventually a cure for this devastating neurological illness. "
Dr Richard Grunewald, a consultant neurologist at the Sheffield Teaching Hospitals NHS Foundation Trust who is also on the panel that gives advice to NICE on CFS, said he had reservations about the research.
He said: "The idea that all CFS can be caused by a single virus doesn't sound plausible to most people who work in the field.
"A lot of the symptoms of CFS are not those of a viral infection."
Sir Peter Spencer, chief executive of Action for ME, said: "It is still early days so we are trying not to get too excited but this news is bound to raise high hopes among a large patient group that has been ignored for far too long.