Herpes simplex virus types 1 and 2 and multiple sclerosis.
A hypothesis is proposed which states that multiple sclerosis (MS) is caused by herpes simplex virus type 2 (HSV-2) in persons lacking herpes simplex virus type 1(HSV-1) immunity. It is examined by comparing the epidemiology of these viruses with that of MS. The age at which HSV-2 infections are first detected, as well as the age of peak incidence are similar to those for MS. If age-specific HSV-1 immunity data from various places is compared, an inverse relationship of immunity levels with latitude is apparent, and may be related to the effects of ultraviolet irradiation on HSV-1 activation and transmission rates. Prior HSV-1 immunity, if related to subsequent risk of MS, would be protective. This hypothesis also appears to be consistent with the generalisations concerning effects of migration on subsequent rates of MS, and with certain other features of MS epidemiology.