Does delay in acquiring childhood infection increase risk of multiple sclerosis?

Multiple sclerosis (MS) appears to be more common in technically advanced countries than in underdeveloped regions, and migration from one area to another at a young age affects the risk of acquiring MS. One way of explaining both the peculiar frequency distribution and the effect of migration while young is to postulate that an infection early in life decreases the chance of central demyelination. However, no specific infection has been implicated consistently. Alternatively, an aberrant host response to infection in childhood might induce central demyelination. Thus, the aberrant host response could be age-dependent. In seeking associations between age of infection and risk of MS, we observed a direct relationship: where childhood diseases were acquired early in life, the frequency of MS in that population was low; where childhood diseases tended to occur nearer adolescence, MS frequency in that population was high. Since immune responsiveness to antigenic challenges matures through early adolescence, we reason that early infection might be protective and that delay in acquiring childhood infections might increase the risk of developing MS. Indeed, in experimental models, the chance of inducing chronic relapsing central demyelination is increased by using adolescent rather than newborn or mature animals. In this paper, epidemiologic evidence showing the strong association between age of infection and risk of MS is presented.