In general it is thought that MS is caused by a combination of factors:
Response to infection/virus
Scientists suspect that MS may be indirectly linked to a virus or bacteria. Researchers are currently studying if genetically predisposed individuals have a hyperactive immune response to a virus or bacteria, and this immune response leads to MS. As of yet, no virus or bacteria has been identified, and there is no evidence that MS is contagious. However, the clear involvement of the immune system in MS indicates this path of research may be promising.
MS is not directly inherited, but it is clear that genetic factors play a role in who gets the disease.
The results of studies of identical twins have mainly brought this to light. If an illness is completely hereditary it should always affect not just one of the twins but the other as well. Non-identical twins have the same risk as that of any other sibling, but the identical twin of a patient with MS has a much higher risk of getting the disease Children and siblings of PwMS are at higher risk of getting the disease than someone who has nobody in the family with MS.
In closing, genetics are clearly involved, but the fact that the identical twin of a PwMS does not always get the disease tells us that more than genetics is involved.
Looked at worldwide, we see clear differences in the occurrence of MS: it mainly affects people of Caucasian descent. This could also point to a hereditary factor among the causes of MS. At the moment, for some hereditary diseases we know that it is possible to point to the exact place where the hereditary factor can be found in our genetic material, the chromosomes. Unfortunately, scientific research is not yet so advanced as far as MS is concerned.
The place where you grew up also plays a role in determining who is more likely to get multiple sclerosis. MS is more common in cooler areas of the globe, particularly, in Europe, North America and Australia.
The differences are not as great as we used to think but we do know that in both the northern and southern hemisphere MS is more frequent the further away a country is from the equator. In the past there have been studies on the effects of emigration. Thus MS occurs relatively less frequently in countries such as Israel and South Africa, and relatively more frequently in England. It would seem that older people who emigrate remain as likely to get MS as if they had stayed in their home country.
But children who emigrate become as likely to get MS as people who have always lived in the country the children emigrate to. There seems to be a turning point somewhere around the age of fifteen that influences the risks of getting MS. Because of this it is thought that the environment people live in could have something to do with the development of MS.
No one knows exactly how many people have MS. According to the World Map of MS prevalence there are over 2.5 million people worldwide, 400 000 in the United States and approximately 5 000 people in SA.
In southern Europe around 50 people in 100 000 have the disease; in parts of Scotland the figures are 402 people in 100,000 (the highest prevalence worldwide) and in the rest of Europe it is around 100 people in 100,000 who have MS. In some populations such as the Eskimos, the illness is unheard of.
There have been some ideas about the influence of diet on the chances of getting MS, but the link is not very clear. To date, there is no evidence of a dietary link with MS.