The etiology of Multiple Sclerosis (MS) is unknown. Existing epidemiologic data suggests that MS can be an infectious disease. MS used to be classified as one of the 'slow infections'--many of these are caused by prions. Prions are small, proteinaceous, infectious particles--distinguished from viruses by the absence of intrinsic nucleic acids. In a contrast to the 'classic' prional diseases (Kuru, Scrapie or Creutzfeldt-Jacob Disease) that in CNS affect primarily neurons, the 'target' cell in MS is an oligodendrocyte. This may explain differences in disease presentation. This paper presents a pathophysiological model of MS based on the assumption that MS is a prional disease. Processes leading to the demyelination in Multiple Sclerosis seem also to involve lymphocytes, astrocytes and macrophages as well as the interferon system.