The etiology of multiple sclerosis is, at present, not definitely known, but genetic and environmental factors play a role in its causation. Environmental causes have a predominant impact. Epidemiologic research has contributed considerably to the identification of external risk factors in this multifactorial setting, but methodological constraints still play a major part. Viral and other microbial agents have drawn much attention, although none of them is a necessary condition for the disease. This is true also for the Epstein-Barr virus, for which most data, including prospective data, supports a role in the majority of multiple sclerosis patients. In parallel, the hypothesis is still attractive in that it is not the virus per se, but rather more the age when it infects the human being that is the crucial matter. Other risk factors, such as tobacco smoking and vitamin D deficiency, which have immunomodulating properties, may also play some role, although the latter is not compatible with all data of the descriptive epidmiology of multiple sclerosis. Diet might be of considerable importance, all the more since multiple sclerosis can be ecologically attributed to a certain food patterns and is inversely associated with others (e.g., the 'Mediterranean diet'). The hypothesis that the preservation of meat by nitrite and wood smoke plays a role, and the protective influence of a fish and, possibly, a vegetable diet, are supported by some studies, but methodological constraints limit, at present, definite conclusions. A new avenue is the search for an interaction between genetic and environmental causes, and also between several environmental factors that might lead to new approaches for prevention and, perhaps, for the treatment of multiple sclerosis.
