The cause of MS is still unknown – scientists believe the disease is triggered by as-yet-unidentified environmental factor in a person who is genetically predisposed to respond.The progress, severity and specific symptoms of MS in any one person cannot yet be predicted. Most people with MS are diagnosed between the ages of 20 and 50, with at least two to three times more women than men being diagnosed with the disease. 

Epidemiological estimatesAlthough more people are being diagnosed with MS today than in the past, the reasons for this are not clear. Likely contributors, however, include greater awareness of the disease, better access to medical care and improved diagnostic capabilities. There is no definitive evidence that the rate of MS is generally on the increase. 

Age: Most people are diagnosed between the ages of 20 and 50, although MS can occur in young children and significantly older adults. 

Geography: In general, MS is more common in areas farthest from the equator. However, prevalence rates may differ significantly among groups living in the same geographic area regardless of distance from the equator. For example, in spite of the latitude at which they live, MS is almost unheard of in some populations, including the Inuit, Yakutes, Hutterites, Hungarian Romani, Norwegian Lapps, Australian Aborigines and New Zealanders — indicating that ethnicity and geography interact in some complex way to impact prevalence figures in different parts of the world. Migration from one geographic area to another seems to alter a person’s risk of developing MS. Studies indicate that immigrants and their descendants tend to take on the risk level — either higher or lower — of the area to which they move. The change in risk, however, may not appear immediately. Those who move in early childhood tend to take on the new risk themselves. For those who move later in life, the change in risk level may not appear until the next generation. While underlining the complex relationship between environmental and genetic factors in determining who develops MS, these studies have also provided support for the opinion that MS is caused by early exposure to some environmental trigger in genetically susceptible individuals. 

Gender: MS is at least two to three times more common in women than in men, suggesting that hormones may also play a significant role in determining susceptibility to MS. And some recent studies have suggested that the female to male ratio may be as high as three or four to one.