* See section on Mortality.
1889 up to this time-may be explained by a successive lessening of the susceptibility of the population, due to their immunization from the preceding attacks of the disease.
But we must be careful not to wish to prove too much with this "immunity of the population." New York, Copenhagen, London, and numerous places in England (see Parsons' report) were severely affected each time, with very marked morbidity (see section on Immunity and Morbidity), in each of the three successive principal epidemics.
The immunity acquired after an attack of influenza unquestionably plays an important epidemiologic part. But, in addition, another important explanatory factor exists, namely, the gradually increasing diminution of the vis contagii, that is, of the "virulence of the contagious material from epidemic to epidemic."
Wutzdorff thinks that the supposition of the lessened virulence of the later epidemics is not consistent with their notoriously greater malignancy (mortality). We cannot admit this.
A germ may become less virulent in the sense of being less contagious, but may become more virulent in the sense of being more pathogenic, especially in the case of the dangerous " mixed infections." At the beginning of the pandemic of 1889 the vis contagii was, as is well known, enormous; the vis morbi, on the other hand, was so slight that it gave rise to ludicrous names ("influenza dinners"). In the later epidemics the vis morbi became more pronounced with the decrease in the vis contagii, owing, perhaps, to the weakened influenza germs entering into closer symbiosis with other pathogenic microbes.
But the increasing immunity of the population and the decreasing virulence of the germ do not easily explain in a satisfactory manner the above mentioned facts and many others which occurred in the study of the peculiarities of the after epidemics. To illustrate this we will take one example, namely, the interesting and important question of the cause of the lessened communicability of the disease in later epidemics.
As before mentioned, the most striking instance is seen in the entire immunity of the greater part of the European Continent in the spring of 1891, at the time of the great North American and English epidemic. If we inquire closely into this epidemic, we see that in the countries affected by the disease communication still played a slight role in the transmission of the pestilence.
In the descriptions all the well known phenomena of the first pandemic are seen again. The infection from person to person, the importation of influenza by one affected with the disease into an immune family, into a house, an insane asylum, a village, etc., all occur. But the numbers of these examples are infinitesimally small when compared with the innumerable ones caused by the pandemic. Viewed alone, influenza still appears to be as contagious as it was at the time of the pandemic. Why should not the disease be disseminated in all directions, as it was then?
We have already mentioned the slight morbidity of the postpandemic epidemics. The danger of importing the contagion from a to b is proportional to the number of sick in a and the susceptibility to infection of persons in b. If, for example, in the first pandemic 50 influenza patients from a, all infected with germs having their full virulence, had arrived in a large city, b, which was highly susceptible, contagion and a further spread of the disease would have followed rapidly. If, on the contrary, in the spring of 1891, on account of the lessened morbidity, there were in a only ten influenza patients, only a part of whom were infected with germs of full virulence had arrived in b, which in the mean time had acquired immunity, it is probable that the disease would have spread but very little. In this way we can explain the exemption of Germany and France in the spring of 1891 at the time of the North American-English epidemic.
Through the considerably severer character of the disease in the after epidemics the number of slightly affected persons able to travel was less, with consequent decreased transportation of the disease. Moreover, the fear of influenza in general was greater, so that the patients with but a mild attack took better care of themselves and remained at home. Nevertheless, the epidemiologic character of the after epidemics must be designated as peculiar. A completely satisfactory explanation is not forthcoming even to day.
The tangled knot becomes looser, without, however, becoming disentangled, if we glance over the disconnected geographic picture of the after epidemics. A detailed study of them reveals the following facts:
The epidemics which followed the primary pandemic arose from the germs which remained. They are, for the most part, autochthonous, local or endemic epidemics, which are but rarely genetic; that is to say, caused by the transmission of the germ successively from place to place. Contagion takes place from person to person to a slight degree, and it may even be carried from place to place; but, as a whole, influenza no longer follows trade routes. The role which communication played in the pandemic is taken in the after epidemics by time and location (that is, the place where living germs have remained. We by no means incline to a "contagious miasmatic" or " locality " hypothesis, which, in the light of our present knowledge, would be ridiculous. But even confirmed contagionists, when explaining the trailing epidemics, are right in using the expression, " the residual germs," although they do not give the reason why these germs remained. We will not discuss this question. Those contagionists who deny a protracted existence of the influenza germ in the external world, because the bacillus is extremely sensitive to drying and all other external influences, and because no permanent form is as yet known, believe that the germ continues to thrive in an attenuated form in the nasopharynx of individual patients. But the reasons why development of these endanthropic attenuated germs should suddenly become virulent again and cause a local epidemic, are as little known to the extreme as to the moderate contagionists, who consider it possible that the influenza germs may lie dormant outside of the human body in a permanent form which is as yet unknown.