GENERAL INTRODUCTION

Although many theories and opinions have been formed about thc etiology and character of multiple sclerosis (MS) (MCALPINE et al. 1955, 1965) the most recent are the theories that MS is due to an autoimmunological process or that this disease is a ”slow virus” infection with a long latent phase (e.g. TOURTELLOTTE & PARKER 1968). These hypotheses of an infectious etiology and allergic pathogenesis are not mutually exclusive ( KURLAND et al. 1965). Epidemiological investigations on the distribution of multiple sclerosis have uncovered many features of this disease (ACHESON 1965). It has been possible, on the basis of these investigations, to calculate a possible critical starting point for the process (KURTZKE 1968 a). On the other hand, the epidemiological investigations have built up a framework which is necessary for studying possible etiological factors (KURLAND 1960). Countries with a highly developed social welfare system including the firm right to a sickness and disability pension and good medical facilities are excellent subjects for intensive and systematically sampled epidemiological investigations into MS ( KURTZKE 1967 b) . I n Scandinavian countries reports on the prevalence of MS have been published from the 30’s onwards (GRAM 1934). I n Finland, the first report was a preliminary work for this study (PANELIUS 1965). This was followed by investigations, in cooperation with others, into the prevalence and mortality of multiple sclerosis on the national level in Finland. This study was based on information from the National Pensions Institute and from hospitals, without a personal examination of the patients by the authors (RINNE et al. 1966, 1968). These studies showed that the south-west part of Finland formed a region with a high prevalence rate of MS. This region was selected for further studies in the present work because the number of MS patients was suitable for a personal examination of the patients. For this reason, the determination of the prevalence rate may be taken to be as exact as possible. The clinical investigation and interview and the selection of the finally approved MS population were made personally in order to get the best possible information about the distribution of MS. Special attention was paid to getting the most accurate control pairs for the MS patients in order to test the importance of some exogenic factors, especially the importance of some infectious agents in the etiology of the disease.