The effects in the body caused by a child fighting off a virus can lead to ME, scientists have discovered.
Abnormalities were found in the white blood cells of children with Myalgic Encephalomyelitis, pointing to the fact that they had been fighting off an infection, research from the University of Dundee found.
Scientists examined a total of 25 young people suffering with ME, which is also referred to as chronic fatigue syndrome, from across the country between the ages of 10 and 18, while a group of 23 youngsters of a similar age in a control group were also assessed.
ME Research UK and The Young ME Sufferers Trust funded the study, which took place over 18 months. The Archives of Paediatrics and Adolescent Medicine journal published the findings.
Dr Gwen Kennedy, who led the team at the University of Dundee, said: "These results are of great importance; not only do they show an underlying, detectable defect in the behaviour of the children's immune cells, they also confirm our previous findings in adults."
Professor Jill Belch, head of the unit at Ninewells Hospital in Dundee, where the study was undertaken, told BBC Radio Scotland's Good Morning Scotland programme: "The findings that we have shown are those that are commonly seen when somebody has a virus or an infection.
"For example, if you have a virus or a bacteria your white blood cell will swallow the particle and then release chemicals to kill it.
"What we found was an increased amount of these killer chemicals and the white blood cells don't last as long, which is a sign that they've been very active. So this is a definite physical sign of ill-health."