MS is thought to affect more than 2.1 million people worldwide. While the disease is not contagious or directly inherited, epidemiologists—the scientists who study patterns of disease—have identified factors in the distribution of MS around the world that may eventually help determine what causes the disease. These factors include gender, genetics, age, geography, and ethnic background.
Patterns in the Distribution of MS
- As in other autoimmune diseases, MS is significantly more common (at least 2-3 times) in women than men. This gender difference has stimulated important research initiatives looking at the role of hormones in MS. Read more on autoimmune diseases.
- MS is not directly inherited, but genetics play an important role in who gets the disease. While the risk of developing MS in the general population is 1/750, the risk rises to 1/40 in anyone who has a close relative (parent, sibling, child) with the disease. In families in which several people have been diagnosed with MS, the risk may be even higher. Even though identical twins share the same genetic makeup, the risk for an identical twin is only 1/4—which means that some factor(s) other than genetics are involved.
- While most people are diagnosed between the ages of 20 and 50, MS can appear in young children and teens as well as much older adults. Studying the disease in different age groups may help scientists determine the cause of MS and explain why the disease course differs from one person to another. Important questions include why the disease appears so early in some children and why people who are diagnosed after age 50 tend to have a more steadily progressive course that primarily affects their ability to walk.
- In all parts of the world, MS is more common at northern latitudes that are farther from the equator and less common in areas closer to the equator. Researchers are now investigating whether increased exposure to sunlight and the vitamin D it provides may have a protective effect on those living nearer the equator.
- MS occurs in most ethnic groups, including African-Americans, Asians and Hispanics/Latinos, but is more common in Caucasians of northern European ancestry. However some ethnic groups, such as the Inuit, Aborigines and Maoris, have few if any documented cases of MS regardless of where they live. These variations that occur even within geographic areas with the same climate suggest that geography, ethnicity, and other factors interact in some complex way.
Are the Numbers of People with MS Increasing?
While more people are being diagnosed with MS today than in the past, epidemiologists have found no evidence to suggest that the disease is on the increase. More likely explanations include a greater awareness of the disease, improved medical care, and more effective tools for making the diagnosis. In addition, the availability of effective treatments makes physicians more likely to communicate the diagnosis to their patients.
What are MS Clusters?
A "cluster" of MS can be defined as the perception that a very high number of cases of MS have occurred over a specific time period and/or in a certain area. Such clusters of MS—or of other diseases where clusters are occasionally reported—are of interest because they may provide clues to environmental or genetic risk factors which might cause or trigger the disease. So far, cluster studies (in the Faroe Islands, Galion, OH, DePue, IL, and El Paso, TX, among others) have not produced clear evidence for the existence of any causative or triggering factor or factors in MS. Read more information about MS Clusters.