Infection
Due to the regional distribution of MS, another theory that has arisen is that the disease is caused by exposure to a bacteria which thrives in the cool environments found more distally from the equator. This factor would serve to provide an explanation as to why the disease's incidence in young migrants reflects that of the countries they move to rather than where they've come from. The hygiene hypothesis suggest that failing to be exposed to pathogens in early life may trigger MS upon re-encounter in later life. Another hypothesis relating to infection is the prevalence hypothesis. This suggests that MS is caused by a bacteria whose infection is asymptomic in the vast majority of the population but does have the potential to trigger MS in rare situations.