Between the occurrence of the first known case of influenza and the time of the first very definite increase in influenza incidence in a community, which interval may be termed the invasion period, there is as a rule two weeks. During this period, of course, more and more cases are occurring, but remain usually sufficiently isolated to attract no public notice. From this point the epidemic develops very rapidly and reaches its peak, usually within two or at most three weeks. In another two or three weeks the incidence has fallen away nearly to normal. The epidemic period comprises from four to six weeks, or, including the invasion period, an entire duration of six to eight weeks. This is the picture produced in a community by a primary uncomplicated epidemic of influenza. Greenwood well describes the salient features of a primary epidemic as “first a rapid and quasi-symmetrical evolution, and second, a frequency closely concentrated around the maximum.” In other words the duration is short, the rise to a peak rapid, and the subsequent fall equally rapid. He showed that in the July and August, 1918 epidemic in Great Britain nearly 80 per cent. of the total incidence in the localities studied was grouped within three weeks time. His curve corresponds so well with that of the Munich epidemic that he is able to superimpose them (Chart I). The rapid rise to a peak, almost explosive in character, more characteristic of this disease than of any other, is to be explained by the high degree of invasiveness of the organism, by the short period of incubation, by the fact that many of the sick continue at their work, thus spreading the disease, and by the non-immunity of large masses of people, together with the fact that the transmission of a respiratory infection is accomplished much more easily than is any other type of infection.