Seven weeks of the 1920 epidemic in Detroit killed 0.20 per cent. of the population, two out of every one thousand people. A similar period at the beginning of the epidemic of 1918 witnessed the death of 0.17 per cent. of the population. This was a smaller number, but the epidemic at this time had not completed its course, and continued to be more or less prevalent for twenty-one weeks, resulting finally in the death of 0.28 per cent. of the population. The recurrent epidemic was more highly fatal, but, being of shorter duration, Detroit actually suffered less from it. SPREAD IN COUNTRIES AND CONTINENTS. The spread of influenza is usually not limited to a single community. Almost invariably it will travel on to another locality, carried thither by human intercourse, and will there build again a local epidemiologic picture more or less modified by changes in the environment and changes in the virulence of the virus itself. _Spread, in primary waves._—Reference to the table of epidemics in history will show that in many of the epidemics and in most of the widespread epidemics and pandemics there appears to have been a definite, clearcut, direction of spread from one locality to others. In the recent literature there has appeared considerable discussion concerning the site of origin, the endemic focus of pandemic influenza. Briefly the question raised is as to whether there are single or multiple foci. We will for the time ignore this perplexing question. In either case, after the influenza virus has once attained such communicability as to produce a pandemic it does follow a direct course over countries and continents. This may be followed in resumé in our table. The disease does not at any time spread more rapidly than the available speed of human communication between the areas affected. If influenza does appear simultaneously in two widely separated communities without having been brought there from a common source it must be that it arose spontaneously from simultaneous increase in virulence of the virus in those localities.