Doctors have found more evidence to suggest that children who develop glandular fever may have an increased risk of multiple sclerosis.
Glandular fever is very common
Doctors in Canada carried out tests on 120 children, 30 of whom had MS.
They found those with MS were twice as likely to be infected with the virus that causes glandular fever.
Writing in the Journal of the American Medical Association, they said the findings suggested the virus may play a role in the development of MS.
Previous studies have suggested that there may be a link between Epstein-Barr virus and MS.
However, it has been difficult to prove, not least because so many people have had glandular fever.
The Epstein-Barr virus is a member of the herpes virus family. About 95% of adults show signs of being infected with it by the time they are 40. Most have been infected by the age of 20.
This latest study is one of the first time doctors have carried out tests on children to see if those with MS are more likely to have had the virus than healthy children.
Dr Brenda Banwell and colleagues at Toronto's Hospital for Sick Children carried out tests on 30 children with MS and 90 healthy children, with an average age of 13.
They found antibodies in the blood of 83% of those with MS, showing they had been exposed to the Epstein-Barr virus. This compared to just 42% of the healthy children.
The doctors said the findings suggested there may be a link between the virus and MS. "The results suggest an association," they wrote.
Multiple sclerosis affects about 2.5m people around the world. The disease attacks the nervous system.
Initially it causes loss of balance, reduced vision and bouts of localised paralysis. Eventually, patients may become totally paralysed.
Scientists believe the condition is caused by a chemical in the body called interferon gamma.
Under normal circumstances this chemical helps to activate the immune system to attack foreign invaders, such as viruses and bacteria.
'Trigger not cause'
In multiple sclerosis patients interferon gamma causes the immune system wrongly to identify body cells are foreign invaders.
As a result, the myelin sheath coating nerves in the brain and spinal cord is destroyed by mistake.
Transmission between nerve cells then slows down and becomes irregular.
Some scientists believe that interferon gamma may see antibodies from the Epstein-Barr virus as a potential attacker, causing it to damage nerve cells.
The MS Society welcomed the study but said further research is needed.
"It is another piece of useful research," said a spokesman.
"It is further indication that viruses may have a role to play in triggering MS.
"However, it is talking about a trigger rather than a cause. The number of children with glandular fever who go on to develop MS is really very small."