The main still unexplained aspect of the epidemiology of poliomyelitis is the question, whether for the genesis of epidemics only the spread of the viruses and the immunity status of the population are responsible or whether there are other factors necessary to start an epidemic. The preference of the disease for the summer and autumn months in epidemics as well as in the interepidemic periods is well known. In this polio season also the viruses circulate more widely in the community. This could be demonstrated by Fox e. a. (1) in the healthy families which were under constant supervision for the presence of polioviruses. From sewage polioviruses could be isolated only in summer and autumn (Kelly e. a.) (2).The poliomyelitis accident in Idaho (3) caused by the Cutter vaccin containing live virus did not grow out to a large scale epidemic presumably because it happened in the unfavourable month of April.There can be two explanations for the seasonal predisposition for poliomyelitis epidemics :1. Climatic conditions could favour the spread of the viruses and thereby the frequency of the disease; 2. the weather could influence the resistance of the host or the aggressiveness of the parasite.With these problems in mind a study was made of the spread of the poliovirnses and the immunity status in the families of poliomyelitis patients. MaterialFrom Dec. 1956--Sept. 1958 266 cases of poliomyelitis were notified in the Netherlands of whom 53 (20%) were nonparalytic. Our study was limited to 104 patients of whom 15 suffered from a non-paralytic form (15%). These patients belonged to 101 families and in 74 the diagnosis was confirmed by

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