The author holds that the infrequency of immunity is a most important factor in the production of this type of outbreak. The mode of transmission of influenza is the same as that of other respiratory diseases. The infectivity is probably no greater than that of measles, although that indeed is relatively great. The means of transmission are presumably the same in each. Were we able to develop an immunity for influenza of as high degree and permanence as we possess against measles, pandemics of influenza would disappear. We wish to emphasize that the primary type of curve is a phenomenon not peculiar to influenza, but that under certain circumstances it may be found in other infectious diseases, and that it would be found more frequently in the other diseases if the immunity developed against them was of as short duration as it appears to be against influenza. If, for example, measles were to break out in a large group of individuals, none of whom had had the disease, the type of curve would be the same. We will produce evidence supporting our theory under another subject. Of course, other factors such as short incubation period and unusual opportunities for spread through mildly ill individuals play a not unimportant role. CHART I. [Illustration: The curves of incidence of influenza in Munich, and of deaths in London during the 1889 and subsequent epidemics. (_Greenwood._) ] _Secondary type of epidemic._—There is a decided difference between the curve of a primary wave as it appears in the onward rush of a new pandemic and that of a secondary wave occurring at a greater or less interval following the primary spread. A secondary epidemic affects, according to Greenwood, a relatively small proportion of the population, is slower in reaching its maximum, and thereafter declines slowly and irregularly, more slowly than it increases. The distribution of the curve is less symmetrical and there is less concentration around the maximum. A secondary epidemic may be characterized by a much higher fatality than a primary one. We believe that the configuration of a secondary type of wave is due chiefly although not entirely to a certain degree of residual immunity in a large number of individuals remaining from the first spread. There is a striking similarity between Chart I and Chart XXVIII, the latter showing the measles incidence in epidemics among rural or chiefly non-immune troops in the United States army. Chart XXIX shows a similar epidemic among urban or chiefly immune individuals. Here the curves correspond more to those of a secondary type of influenza epidemic. Thus we see that, in the absence of immunity, other infectious diseases may produce the primary type of curve, and that this curve is not a feature of influenza alone.