Page last updated at 09:11 GMT, Friday, 9 October 2009 10:11 UK
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.
One can read this article here.