The analysis of an epidemic of influenza in the New Zealand division of the Royal Navy confirms the following hypotheses which were suggested by previous studies of the same and other diseases.

(1) The type of influenza in different places, especially if they are more or less isolated areas, such as islands or ships, tends to assume special characters as regards infectivity, virulence, and clinical type1. In this respect the influenza virus, or viruses, are only following a well-known biological “law,” namely, that isolation tends to encourage the evolution of new varieties and species. A classical example of this “law” is found in Darwin's reference to the fact that each island of the Galapagos Archipelago has its own special flora and fauna—similar to but yet different from that of the other islands.

(2) One wave of influenza, even if of a different type, will confer considerable immunity to a subsequent wave not only on the victims attacked, but also on those who apparently escaped infection during the first wave.

(3) The brunt of the morbidity of influenza (and other infections) is borne by recruits and junior ratings, not because they are, on the whole, younger, but because they have had less time than the senior men to adapt themselves, by means of auto-vaccination, to the bacterial environment of crowded ships or barracks.

The effect of bacterial adaptation to environment is often overlooked. Hence other factors, such as physical fitness, are often given more credit than they deserve as preservers of the public health.

(4) Senior ratings mixed with recruits suffer a higher morbidity from infectious disease than they do under similar circumstances in the absence of recruits. This statement, if true, should have a most important practical bearing on military medical administration. The phenomenon is probably due to increased velocities of infection (due to the higher infectibility of the recruits) breaking down the herd immunity of the senior men.

(5) In this epidemic New Zealanders suffered more from influenza than the British sailors not, as usually supposed, because of the mere fact of being born in New Zealand, but because all the recruits and the bulk of the junior ratings were New Zealanders.