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Environmental
The map below has two red lines running horizontal that are set at about 40° latitude north and south. The areas above the north latitude and below the south latitude are the spots with an overall higher occurrence of multiple sclerosis (MS). Notice that the majority of North America and Europe to Russia are included as well as the bottom tip of South America and Australia. The areas inside both 40° lines still have cases of MS, but at a lower overall rate of about 5 to 1.

Data has shown that MS is especially common in Scotland, Scandinavia, and throughout northern Europe. In the U.S. the prevalence of MS is higher in whites than in other racial groups. Studies have shown that it's more common in certain parts of the world, but if you move from an area with higher risk to one of lower risk prior to adolescence, then you acquire the risk of your new home location. This would suggest that exposure to some environmental agent encountered before puberty may predispose a person to MS.

MS is a disease of temperate climates, meaning the zones in a range of latitudes between 40° and 60/70°. Not as hot as the subtropical climate and milder than the polar climate. In both hemispheres, its prevalence increases with distance from the equator.

There are many theories as to why MS is more prominent in the areas with greater temperature variations versus the areas with a more constant temperature. There are temporal and geographic variations in disease risk, and that risk of disease may be affected by migration between regions. Numerous potential causal factors including infection, immunizations, physical and emotional stressors, climate, diet, and occupational exposures have been studied using various observational study designs.

The Blue line indicates the equator and the two Red lines indicate the 40° latitude mark

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The Blue line indicates the equator and the two Red lines indicate the 40° latitude mark

It's possible that the colder temperatures cause viruses to spread more rapidly due to changes in the populations habits. Now if a virus is one of the key components to MS, and the colder weather keeps the population more closed in, they may be more likely to pass a virus along, similar to how the influenza virus is passed in the winter months. With a somewhat large number of the world's population near and above the 40° north latitude line, it could also be the close proximity that these groups are to each other all year long. These areas where involved heavily with the early industrialization of the world, including heavy refining of metals, petroleum, and the subsequent and various pollution - most are still involved.

The geographic distribution of MS and the change in risk among migrants also has provided evidence for the existence of strong environmental determinants of MS. This can also mean that environmental can also include the various regions differences in diet and other behaviors.

It's also possible that the consistently warmer areas closer to the equator allow those populations' to stay a bit further apart from each other. This could possibly reduce the spreading of these types of viruses. It's also possible that the later industrialization of these areas has not exposed the populations to the environmental agents over the period of time needed for the damage to show up as MS.

Could it be possible that the world's desire for development has given yet another consequence to our constant need for more? Could it be possible that we are the ones who are responsible for this and probably many other conditions? Can MS be another consequence of global warming?

Some scientists think the reason may have something to do with vitamin D, which the human body produces naturally when the skin is exposed to sunlight. People who live closer to the equator are exposed to greater amounts of sunlight year-round. As a result, they tend to have higher levels of naturally-produced vitamin D, which is thought to have a beneficial impact on immune function and may help protect against autoimmune diseases like MS.

Some populations like the Inuit never get MS. Native Indians of North and South America, the Japanese, and other Asian peoples have a very low incident rate with MS. This would seem to indicate that a genetic barrier may exist in these groups of people that would protect or prevent MS from developing.

There have also been "epidemics" of MS, such as the time it occurred in a group of people living off the coast of Denmark after WWII, suggesting an environmental cause.

For those who smoke, it's been shown that they have a 40 to 80% increased risk of MS compared with those who don't smoke. A study published in Oxford Journal's Brain of more than 2,000 people with and without MS showed that smoking may not only increase the risk of developing MS, but in those who already have MS, smoking may speed progression of the disease and worsen symptoms.