Factors bearing on the maintenance of paratyphoid in an endemic state are discussed. There was no evidence of any increase nor any dearly demonstrable proof of a decline in virulence of the causative organism. This persisted within the breeding stock and it is suggested that the sows constituted the chief focus for dissemination of the organism to their young and from these to the population at large. Evidence is presented that the carriage of B. paratyphi in the feces was of relatively short duration. Fecal carriage of B. paratyphi was commonly associated with a localization of the organism in the spleen. Since it is obvious that some factor or factors must have changed in the transition from epidemic to endemic phase in the presence of younger generations, the hypothesis is tentatively presented that the transition from epidemic to endemic phase is due to a combination of the weeding out of individuals of low natural resistance with a gradual adjustment of the invading organism to the population on a lowered level of virulence.

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