Encephalitis lethargica and influenza. I. The role of the influenza virus in the influenza pandemic of 1918/1919.

An investigation of the characteristics of influenza epidemics in the nineteenth and early twentieth centuries was undertaken, principally in order to analyze the role of the 1918/1919 influenza pandemic in the etiology of encephalitis lethargica. Expectations regarding a future influenza pandemic derive principally from experiences in the 1918 epidemic. It is proposed that this pandemic was atypical with respect to many of its features, and that these have not been appropriately regarded in mapping expectations and responses of a future pandemic. Both a longer historical viewpoint (incorporating knowledge from all major nineteenth and twentieth century epidemics) and closer examination of individual epidemics at the town level is essential for producing an accurate picture of the challenge.