A study on children has found further evidence that ME, or Chronic Fatigue Syndrome, could be caused by a virus.
Scientists at the University of Dundee study found abnormalities in the white blood cells of children with ME/CFS, suggesting they had been fighting off infection.
ME (Myalgic Encephalomyelitis), also known as Chronic Fatigue Syndrome (CFS), causes debilitating tiredness.
About 150,000 people in the UK have ME/CFS, 15,000 of whom are children.
The condition is characterised by physical and mental exhaustion following normal activities. Symptoms can include muscle pain, sore throat, tender lymph nodes, multi-joint pain and headaches.
In the study, funded by ME Research UK and The Young ME Sufferers (Tymes) Trust, 25 children aged between seven and 14 with ME/CFS were assessed, along with 23 children of a similar age in a control group.
The report, published in the Archives of Paediatrics and Adolescent Medicine, said abnormalities were found in the blood of all the children with ME/CFS.
The results were similar to those previously identified in adults with the condition.
Samples taken from youngsters with ME/CFS contained higher than normal levels of free radicals - molecules that can damage cells, tissues and organs.
A much greater number of neutrophils, the most common type of white blood cells, were also found to be at the end of their lifecycle.
The report said the high turnover of neutrophils indicated the body's need to fight infection.
There is a continuing debate among scientists over whether ME/CFS is caused by a virus.
Several studies in adults have found evidence of a virus in people with the condition, but so far research has not proved conclusive.
Some doctors have said that the idea that different types of chronic fatigue are all caused by a single virus is not plausible.
Professor Jill Belch, an expert in vascular medicine at Ninewells hospital in Dundee who led the latest research project, said: "What we've found are blood changes that suggest chronic inflammation.
"This is important because it's showing an abnormality that we might be able to devise a treatment for, but it's also important because some people do suggest that ME is a disease of the mind and here we are showing that it is a disease of the body."
Dr Neil Abbot, of ME Research UK, said it was " fascinating to discover evidence of a persistent or reactivating viral infection".
"Although the cause of ME is unknown, more than half of all patients say their illness started with an infection," he said.
"The study undoubtedly adds greater scientific weight to the existence of a condition which, sadly, many still fail to acknowledge in spite of its severity."
According to Tymes Trust, children with ME can be treated with "scepticism" by the healthcare system.
Jozef Mackie, 14, from Fortrose, near Inverness, was a sporty child who loved skiing, until the symptoms of ME began when he was nine years old.
His mother, Donna, said: "He was the boy whose battery wasn't working very well. He had to take more and more time off school.
"The other children were able to run around and do things after school and Joseph had to come home and lie on the settee."
For two years Jozef was labelled a "school refuser" and told it was "all in his head".
He was finally diagnosed with ME when he was 11.
"I haven't been treated very well," said Jozef. "I haven't been believed. They just think I'm putting it on.
"It's sore to get in and out of the car and I can't walk long distances."
Jane Colby, from Tymes Trust, added: "The medical profession must now take the consequences of ME in children seriously, and research into prevention and treatment must be given a high priority."