Our own experience was quite similar. We have divided the whole period from March, 1918 to March, 1920, into two portions separated at August 1, 1919. In the first portion we have knowledge of but four individuals suffering from what the records would indicate to be two genuine attacks of influenza. Similarly, five individuals appeared to have had two attacks within the second interval. These are to be contrasted with a total incidence in the fall and winter of 1918–1919 of 1,971 cases, and in the winter of 1919–20, of 965 cases. Among the total nine individuals the intervals between attacks varied from 26 days to five months. All except one had an interval of one month or over. In two cases there was an interval of one month, in one an interval of two months, in two an interval of three months, in one of four months, and in one of five months. None of the four individuals who had two attacks in the first group of months had a subsequent attack in the second. On the contrary, two of the five suffering two attacks in the second group of months had one previous attack in the first. The second attack, following the first by a relatively short interval tended to be milder than the first. In five out of the entire nine the second attack was milder, in two it was of the same degree of severity, and in only two was it more severe than the first. The order of severity in the two individuals having three attacks each was, in the first, severe, mild, severe; in the second, severe, average, average. Zinsser makes the following remark: “The writer himself believes that he had three attacks during the last epidemic. The first and second were mild ones and the third complicated and therefore severe; and innumerable others with whom he has spoken have had similar experiences.” From a consideration of these reports by divers authorities it is reasonable to conclude that for a period of a few months at least, one attack of influenza protects against a second. As is to be expected, this relative immunity is not of constant duration in all individuals. If there were no lessened susceptibility following an attack we would be faced with the phenomenon of individuals succumbing time and again to rapidly successive attacks of the disease. Such a circumstance is very rare.