Prior to 1870 poliomyelitis occurred only as a sporadic disease of the very young child. During the 1870s in Europe, the 1880s in Scandinavia, the 1890s in the northern part of the United States and the 1930s in the southern part of the United States poliomyelitis took on a new character—occurring in epidemic proportions. The reason for this transition from a sporadic to an epidemic disease is unknown, but of particular interest is the fact that with this transition there has been a gradual increase in the average age of the persons afflicted.
There is recent evidence to support this historical belief that such changes are part of the pattern of the disease. At the present time this evolution seems to be occurring in the larger metropolitan communities of Osaka and Kobe in Japan. During 1938 and 1940 these urban areas experienced for the first time severe epidemics of poliomyelitis,
VAN RIPER HE. POLIOMYELITIS. JAMA. 1947;135(2):74-76. doi:10.1001/jama.1947.02890020004002