The risk of developing Multiple Sclerosis is under the combined influence of environmental and endogenous factors, as shown by epidemiological and genetic studies. Immunological abnormalities are a salient feature of the disease but their myelinotoxic effect is still to be demonstrated. The pathogenesis of the disease remains unknown. The auto-immune theory as well as the infectious theory are arguable but they may, also, be both valid. Multiple sclerosis could be a post-infectious auto-immune disease, "final common way" of common childhood infections. Anyway, no infectious agent is demonstrable inside the central nervous system at the overt disease stage.