Careful etiologic study of 713 patients with disease of the central nervous system of presumed "viral" etiology related forty immunologically distinct infectious agents to the diseases. Specific etiologic diagnoses were established in 73 per cent of these patients, studied between 1953 and 1958. Clinically, disease in each patient could be classified as aseptic meningitis, encephalitis or "paralytic poliomyelitis." Aseptic meningitis, a uniformly benign syndrome, was of diverse etiology, associated most frequently with Group B Coxsackie, mumps, ECHO, LCM and polio-virus infections. Encephalitis, which in this series varied in severity, was also of diverse etiology and was caused by many of the same viruses. Mumps, LCM, arthropod-borne and herpes simplex viruses were most often implicated; death and severe sequelae were more frequent in herpes simplex and arthropod-borne virus encephalitides. The paralytic polio-myelitis syndrome (i.e., lower motor neuron paralysis with no evidence of cortical involvement), on the other hand, was due almost entirely to polioviruses. Enterovirus infections accounted for 41 per cent of all patients studied. Twenty-one different enteroviruses were encountered, although not in equal proportions or constant frequency from year to year. Enterovirus disease, although observed in all months of the year, was more common in late summer and fall. Seasonal variations were also noted for LCM, arthropod-borne and mumps viruses, but probably not for herpes simplex virus or leptospirae. These seasonal variations in virus prevalence were valuable adjuncts to differential etiologic diagnosis in the individual patient.
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