There is no simple single answer to the questions “what causes MS?” or “why do some people get MS?” Over the last century and a half, three important inter-related contributing factors have been recognized: environmental (usually thought to be infectious), immune, and hereditary (genetic) factors. Obviously, it would be impossible to do more than superficially discuss these issues. The most commonly asked questions are addressed.
Population studies have yielded information from which it has been inferred that an environmental factor exists. Persons moving from high-risk to low-risk areas take the risk with them if they move after the age of 15 years. Conversely, if they move before the age of 15 years, they appear to leave the risk behind. This information comes from studies of populations moving from Europe (a high-risk area) to Africa (a low-risk area).
Similar observations have been noted in the populations moving into Israel. These findings, as well as the occurrence of an epidemic of MS in the Faeroe Islands after the “invasion” of those islands by British troupes at the outset of World War II, suggest that an infectious agent is playing a role in MS.