Foremost among the many unknowns in the pathogenesis of poliomyelitis are the factors that determine the type of response to the infection. There is ample evidence that infection with the poliomyelitis viruses is widespread, ultimately involving almost all persons, except those in isolated areas. In most of these persons, the infection is mild, producing no paralysis and usually occuring without recognizable symptoms. Yet a small number of persons respond badly to the virus invasion, with resultant paralysis. Of those with central nervous system involvement, a few show involvement of the vital medullary centers, while the majority show lesions of the spinal cord. The proportion with bulbar paralysis has supposedly increased in recent years and is greater in the older age groups. Very little evidence is currently available, however, as to the factors that determine this localization of the paralysis in the bulbar area.
Among the many factors that have been