Editing Multiple sclerosis (section)

=== Geography ===
[[File:MS Risk.svg|thumb|Geographic risk distribution of MS]]

The prevalence of MS from a geographic standpoint resembles a gradient, with it being more common in people who live farther from the [[equator]] (e.g., those who live in northern regions of the world), although exceptions exist. The cause of this geographical pattern is not clear, although exposure to [[Ultraviolet|ultraviolet B]] (UVB) radiation and [[vitamin D]] levels may be a possible explanation.<ref name="Milo2010"/><ref name="Ward 988–1005"/> For example, those who live in northern regions of the world have less exposure to UVB radiation and lower levels of vitamin D, and a higher risk for developing MS.<ref name="Ward 988–1005"/> Inversely, those who live in areas of higher sun exposure and increased UVB radiation have a decreased risk of developing MS.<ref name="Ward 988–1005"/> As of 2019, the north–south gradient of incidence is still present and is increasing.<ref name="pmid31217172">{{cite journal | vauthors = Simpson S, Wang W, Otahal P, Blizzard L, van der Mei IA, Taylor BV | title = Latitude continues to be significantly associated with the prevalence of multiple sclerosis: an updated meta-analysis | journal = Journal of Neurology, Neurosurgery, and Psychiatry | volume = 90 | issue = 11 | pages = 1193–1200 | date = November 2019 | pmid = 31217172 | doi = 10.1136/jnnp-2018-320189 }}</ref>

On the other hand, MS is more common in regions with northern European populations,<ref name="pmid1897097722"/> so the geographic variation may simply reflect the distribution of these higher-risk populations.<ref name=Milo2010 />

A relationship between season of birth and MS lends support to this idea, with fewer people born in the Northern Hemisphere in winter than in spring.<ref name="pmid19897699">{{cite journal |vauthors= Kulie T, Groff A, Redmer J, Hounshell J, Schrager S |title= Vitamin D: an evidence-based review |journal= Journal of the American Board of Family Medicine |volume= 22 |issue= 6|pages= 698–706 |year= 2009 |pmid= 19897699 |doi= 10.3122/jabfm.2009.06.090037 |doi-access= free}}</ref>

Environmental factors during childhood may play a role, with several studies finding that people who move to a different region of the world before the age of 15 acquire the new region's risk of MS. If migration takes place after age 15, the person retains the risk of their childhood region.<ref name="pmid1897097722"/><ref name="pmid15556803" /> However, some evidence indicates that the effect of moving may apply to people older than 15.<ref name="pmid1897097722"/>

There are some exceptions to the above-mentioned geographic pattern. These include ethnic groups that are at low risk and that live far from the equator, such as the [[Sami people|Sami]], [[Indigenous peoples of the Americas|Amerindians]], Canadian [[Hutterite]]s, New Zealand [[Māori people|Māori]],<ref name="pmid121276522">{{cite journal |vauthors=Pugliatti M, Sotgiu S, Rosati G |date=July 2002 |title=The worldwide prevalence of multiple sclerosis |journal=Clinical Neurology and Neurosurgery |volume=104 |issue=3 |pages=182–91 |doi=10.1016/S0303-8467(02)00036-7 |pmid=12127652 |s2cid=862001}}</ref> and Canada's [[Inuit]],<ref name="Milo2010" /> as well as groups that have a relatively high risk and that live closer to the equator such as [[Sardinian people|Sardinians]],<ref name="Milo2010" /> inland [[Sicily|Sicilians]],<ref>{{cite journal |vauthors=Grimaldi LM, Salemi G, Grimaldi G, Rizzo A, Marziolo R, Lo Presti C, Maimone D, Savettieri G |date=November 2001 |title=High incidence and increasing prevalence of MS in Enna (Sicily), southern Italy |journal=Neurology |volume=57 |issue=10 |pages=1891–3 |doi=10.1212/wnl.57.10.1891 |pmid=11723283 |s2cid=34895995}}</ref> [[Palestinians]], and [[Parsi]].<ref name="pmid121276522" />