The environment and the nervous system.

Multiple sclerosis (MS) is used as the best example to illustrate the interplay between genetic endowment and environmental effects. The former is widely considered as being the more important one, although there is mounting evidence that environmental factors influence genetic predisposition. The vast majority of environmental epidemiological studies have failed for two major reasons: they have lacked biological plausibility, and they have not considered possible environmental factors operative during the putative period of acquisition of the disease. Lack of diagnostic precision in distinguishing among the inflammatory demyelinating diseases has caused confusion. The role of infectious agents remains controversial: there is little evidence in favor of MS resulting from a specific infection, but the effect of viral infections and vaccinations on the immune system of a genetically vulnerable person remains unresolved.