In areas near the equator, MS occurs in fewer than 1 out of 100,000 people. In areas
farther from the equator—such as northern Europe and northern North America—MS
occurs in around 30 to 80 out of 100,000 people.1 When moving south of the equator, the number of people
with MS is less dramatic, but the same trend is seen.
Some evidence suggests that people who move from a high-risk to a
low-risk area before the age of 15 reduce their chances of developing MS.
But the same is true in reverse. In those who move from a low-risk area to
a high-risk area before the age of 15, the risk of getting MS increases. Those
older than 15 when they move to a new area retain the risk associated with
their old area.1
Most experts agree that this unusual relationship between geographic
location and MS suggests that an environmental factor is partly responsible for
causing the disease.
Ropper AH, Samuels MA (2009). Multiple sclerosis and
allied demyelinative diseases. In Adams and Victor's Principles of Neurology, 9th ed., pp. 874–903. New York:
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