In the majority of cases, undoubtedly, the influenza germ enters and settles in the respiratory tract, generally in the upper, nasal, or nasopharyngeal portion, but occasionally also primarily in the trachea, bronchi, or even the alveoli, as shown clinically by the occurrence of a primary influenza pneumonia.
The respiratory tract is probably also the only exit for the bacteria, at least so far as the danger of infection is concerned.
The exclusively gastro intestinal forms of influenza indicate a possible primary invasion of the germ by the stomach and intestinal canal, but it is very likely that in this case also the primary entrance is by the upper respiratory passages, although without any noticeable lesion at the point of entrance. This also is the probable explanation for the frequent purely nervous forms of influenza-that is, of those which run their course without any appreciable respiratory or gastric manifestations.
From the facts just mentioned, certain conclusions logically follow regarding the most important mode of transmission of the contagion. There can be no doubt that a disease which affects the respiratory passages, and particularly the upper portions, in such a striking way, must be air borne, just as the germs of the intestinal infectious diseases, cholera, typhoid, and dysentery, enter principally by the "ingesta " (water and food). While holding this view, we are far from admitting the erroneous miasmatic theory of the panaero dromic distribution of influenza from Russia over the whole earth. At the point of outbreak of influenza the aerodromic convection of the contagion doubtlessly plays an important and even a decisive part. The sensitiveness of the influenza bacilli to drying, which has been proved bacteriologically and is a preliminary condition for the formation of dust, does not disprove this, nor does the aerodromic distribution of the influenza germs in the least affect our belief in the contagionistic theory, which recognizes, at present at any rate, a multiplication of the micro organisms only within the human body.
It is not our intention to enter into the controversy regarding the epidemiologic relation of influenza to other infectious diseases (especially scarlet fever, measles, typhus, diphtheria). It will suffice to quote the opinion which P. Friedrich, after an exhaustive study of the literature on the subject, has formed. He says: "From the sum total of the observations it can be deduced with certainty that there was no mutual correlation between influenza and other infectious diseases during the epidemic of 1889-1890." Wutzdorff arrived at similar negative conclusions regarding the after epidemics, and so did A. Ripperger, in his "historic pathologic studies" concerning the subject under discussion.* Nevertheless, the general rule that during the existence of great pestilences the other acute infectious diseases remained in the background probably held good also at the time of the influenza pandemic.
* "Munch, med. Wochenschr.," 1893, No. 41.