My view of the nature of this disease coincides with that of Dr. Mackenzie; The disease encephalitis lethargica may be defined as an infection the toxic products of which have an affinity for the grey matter of the central nervous system, and so give rise to any of the syndromes of disease of that tissue or to any combination of such syndromes, and which runs a fickle course that may end in recovery, death, or the production of characteristic sequeæ.