Multiple sclerosis and childhood infections.

Migration from an area where MS is common to an area where it is rare (and vice versa) affects the risk of MS, provided migration occurs in childhood. A childhood infection might explain this effect. Therefore, the age pattern of infectious diseases in different regions was examined. A higher proportion of children showed positive titers to many viral diseases early in life in areas where MS is rare compared with those where MS is common. Also, mortality rates from a variety of infectious diseases correlated negatively with the MS mortality. Thus, infection early in life may "protect" against MS, and conversely, later infection, when the immune system has partially matured, may increase risk. MS may be an age-dependent, host-immune response to childhood infection.