Environmental factors in multiple sclerosis: Epstein-Barr virus, vitamin D, and cigarette smoking. 
Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system.  Although its cause is not known, genetic and environmental factors both influence susceptibility to the disease.  From the large array of environmental risk factors that have been examined over the past century, infection with Epstein-Barr virus, ultraviolet light exposure/vitamin D status, and cigarette smoking stand out because of the strength of supporting evidence.  Epstein-Barr virus infection early in life appears to be necessary permissive step for disease development and probably acts in adolescence and early adulthood.  Vitamin D-related mechanisms and cigarette smoking probably modify baseline disease risk and could plausibly do so at several stages of life; smoking appears to negatively affect disease course.  This review summarizes current evidence for these 3 putative multiple sclerosis risk factors and early attempts to develop risk models and examine gene-environment interactions for this complex disease.