Multiple sclerosis cannot be cured, although its symptoms can often be treated. Although it is one of the most common causes of disability among younger people, its causes have been disputed.

Now, a huge study of US military personnel suggests almost all cases of of the condition are triggered by the common Epstein-Barr virus, apparently confirming a three-decade old hypothesis. 

However, what differentiates those who develop symptoms from those who live normally with the virus remains unclear - however childhood obesity, smoking, genetic inheritance and vitamin D deficiency are seen as possible factors. 

Neil Cairns finds out more from Harvard Epidemiology professor Alberto Ascherio and MS sufferer Mick Slater.