environmental factors – of both infectious and non infectious origin –
have been proposed as risk factors for MS. MS is more common in people
who live further away from the equator. Why this is the case is not
clear, but decreased sunlight exposure has been linked with a higher
risk of MS and there is growing evidence that a lack of vitamin D is
linked to increasing prevalence in a range of conditions including MS.
As we get most of our vitamin D through exposure to sunlight, low sun
exposure and subsequent vitamin D insufficiency has been proposed as one
explanation of this effect. This effect may also explain the recent
observation that there is an excess MS risk in people born in April and
May, and a reduced risk in those born in October and November.
microbes (particularly Epstein Barr Virus, EBV) have been proposed as
potential triggers for MS, but none have been proven. Age at exposure to
infection seems to play an important role, and it has been shown that
moving at an early age from one location in the world to another alters a
person's subsequent risk of MS.
Another environmental factor that seems to be strongly associated with MS is cigarette smoking.