Environment and Climate
MS is rare in the tropics and much more common in temperate regions. It is more common in northern Europe than in the Mediterranean countries, though there are exceptions to this pattern. The incidence in Iceland is low but there are pockets of high incidence in Sardinia and Sicily. This seems to be genetic, with Nordic races being more susceptible than Celts or people of Mediterranean origin. MS is also common in the US and Australia.
The environment a person grows up in seems to affect their risk of getting MS. It may be more common in cooler, northerly climates because of the types of viruses that live in those environments. Interestingly, people who move from low-risk to higher-risk climates after the age of 15 seem to remain at the lower risk level, whereas people who move in childhood take on the risk level found in their new country. There is no evidence to link environmental toxins with MS.