Some cases of chronic fatigue syndrome could be due to brain "injuries" caused during the early stages of glandular fever, scientists suggest.
Scientists believe the brain may be 'reprogrammed' by a virus
A University of New South Wales team has followed people with Epstein-Barr virus since 1999.
They suggest those who remained ill after the virus had gone had suffered a "hit-and-run injury" to the brain.
Writing in the Journal of Infectious Diseases, they said the brain appears to keep behaving as if a person is ill.
Epstein-Barr virus causes glandular fever, sometimes known as "the kissing disease".
Symptoms include fever, sore throat, tiredness, and swollen lymph glands.
Most patients recover within a few weeks but one in 10 young people will suffer prolonged symptoms, marked by fatigue.
If these symptoms persist, to a disabling degree for six months or more, the illness may be diagnosed as chronic fatigue syndrome (CFS).
The researchers followed the course of illness among 39 people diagnosed with acute glandular fever.
Eight patients developed a "post-infective fatigue syndrome" lasting six months or longer, while the remaining 31 recovered quickly.
The scientists then looked for signs of the Epstein-Barr virus in blood samples collected from each individual over 12 months.
Professor Andrew Lloyd, of the research team, said: "Our findings reveal that neither the virus nor an abnormal immune response explain the post-infective fatigue syndrome.
"We now suspect it's more like a hit-and-run injury to the brain.
"We believe that the parts of the brain that control perception of fatigue and pain get damaged during the acute infection phase of glandular fever."
He added: "If you're still sick several weeks after infection, it seems that the symptoms aren't being driven by the activity of the virus in body, it's happening in the brain."
The researchers now plan to test their "brain injury" hypothesis by doing neurological tests on the study participants.
Dr Charles Shepherd, medical adviser to the ME Association, said: "This research certainly fits in with other published results showing that glandular fever, along with a number of other common viral infections, is a common trigger for ME/CFS.
"What keeps the illness going remains uncertain, but one possibility is that although the virus disappears, some part of the immune system's response to the triggering infection then affects certain key parts of the brain that are involved in the production of fatigue and the other characteristic neurological symptoms."
Chris Clark, of Action for ME, said: "We would like to know more about this theory.
"It could be an important part of the jigsaw, but probably isn't the entire answer."