The epidemic recurrence of poliomyelitis, with sporadic interepidemic outbreaks which appear to keep the disease alive, and the postinfluenzal epidemics of encephalitis, with their residue of sporadic cases, are still an uncontrolled menace. The factors underlying the epidemiology of these diseases are not completely understood. In these two important diseases, as well as in hydrophobia and experimental herpetic encephalitis of rabbits, for the latter of which specific etiologic filtrable viruses seem to be well established, the histologic changes in the brain are fairly characteristic. They consist essentially of injury to ganglion cells, with perivascular infiltration, chiefly by lymphocytes. Because of the localization of the changes in the central nervous system, a derivative of the ectoderm, Levaditi several years ago proposed the name neurotropic ectodermoses for this group of diseases.
During the last few years an apparent increase has occurred in severe nervous complications of certain infectious diseases and of certain