Scientists believe that MS is a complex disease which is caused by a combination of factors. It's thought that it occurs when someone who has a genetic predisposition to develop MS is exposed to something in the environment, such as a virus or germ, which then triggers the condition.

Studies of people who move from areas where MS is common to areas where it isn't, suggest that this exposure probably happens in early adolescence.

Over the years lots of infectious agents have been blamed for causing MS, but most of these haven't stood up to scientific scrutiny. Recently, EBV, the virus which causes glandular fever, has been linked to MS. Several studies appear to have shown that people with MS have been exposed to EBV and that EBV is active in their bodies during MS attacks.

Could EBV be the possible cause of MS? But EBV is one of the most common viruses in the environment, with up to 90 per cent of the population thought to have been infected by it at some time, most of whom do not go on to develop MS. How helpful is it for scientists to look at the role of EBV in MS?

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