Many microbes have been proposed as triggers of MS, but none have been confirmed.  Moving at an early age from one location in the world to another alters a person's subsequent risk of MS.  An explanation for this could be that some kind of infection, produced by a widespread microbe rather than a rare one, is related to the disease.  Proposed mechanisms include the hygiene hypothesis and the prevalence hypothesis. The hygiene hypothesis proposes that exposure to certain infectious agents early in life is protective, the disease being a response to a late encounter with such agents.  The prevalence hypothesis proposes that the disease is due to an infectious agent more common in regions where MS is common and where in most individuals it causes an ongoing infection without symptoms. Only in a few cases and after many years does it cause demyelination.   The hygiene hypothesis has received more support than the prevalence hypothesis.