Epidemiology, Incidence, and Prevalence
Age: MS can occur at any age. However, 90% of people with MS are diagnosed between ages 16 and 60 with a peak onset of about 30 years of age.
Gender: MS occurs more often in women, at a ratio of approximately 2 to 1.
Race: Caucasians, especially those who were born in temperate areas of the world, such as North America and northern Europe, are more affected by MS than are: Native Americans, African-Americans, Hispanic-Americans, and Asian-Americans.
Geography: Geography is a factor in the incidence of MS. The occurrence of multiple sclerosis in the northernmost states of America is 150 to 200 cases per 100,000 population compared to 50 per 100,000 in the southern states. The frequency drops to 1 case per 100,000 population in equatorial areas. Some studies indicate that the prevalence is reversed in the southern hemisphere, where higher rates are found in the southernmost portions of that temperate zone. Growing up in an area of lower frequency and then moving to an area of higher frequency after the age of 15 years seems to retain the risk of lower frequency area. Moving from a lower frequency area before the age of 15 years confers the risk of the higher frequency area.
Heritability: family history is a known MS risk factor. A recent study by H. Kuusisto, MD investigated the heritability of MS within Finnish twin sets. Kuusisto found that when one monozygous twin is diagnosed with MS, the second twin had a 30% probability to be diagnosed with MS. Similarly, the risk of MS for a fraternal twin of an MS affected individual was 14.3%.
Finally, researchers have advanced various theories involving exposure to a bacterial or viral agent that sets the stage for MS. To date no infectious agent has been correlated with MS. There is also no evidence that MS can be transmitted from one individual to another.