Researchers in Norway believe Chronic Fatigue Syndrome (CFS), also known as
ME, may be caused by a wayward immune system attacking the body.
The illness, the cause of which is uncertain and has no known cure, has attracted significant controversy.
A small study, reported in PLoS One, showed a cancer drug, which inhibited the immune system, relieved symptoms in some patients.
The ME Association said the findings were "very encouraging news".