The numerous instances recorded in literature regarding the direct transmission of influenza from person to person can be only partially substantiated. In many cases other explanations are possible. With an infectivity like that of influenza it is easy to show direct communication among the diseased individuals, but, keeping the possible errors in mincl, if we examine the available evidence, we shall come to the conclusion that there is an enormous number of observations to establish without doubt the fact of direct transmission to the exclusion of all other explanations.
The contagious character of influenza is seen when it breaks out in a large family, in which, as a rule, the remaining members of the family follow the originally affected individual in quick succession. "Mirum est totas civitates repente occupari et ubi unus corripitur aliqua in domo statim singulis ejus familise malum communicari" (Mercurialis, 1580).
The evidence of the above mentioned rule is greatest at the beginning of the epidemic and when it is dying out, and is least during the height of the epidemic, at which time many opportunities for contracting influenza outside of the house exist.
It is, therefore, easily explicable that influenza often enters the household in a vehement and miasmatic manner, simultaneously affecting all. "Non unus vel alter de tota familia sed fere omens eodemmomento lecto decumbere debuerunt" (Juch, 1742).
But there are also exceptions to this rule, for the remaining members of the family, in spite of their intimate contact with the diseased individual, frequently remain exempt. In the English statistics (Parsons' report) many such exceptions are mentioned, and many English physicians even emphasize the fact that, as a rule, there is only one case at a time in a family. At the time of the pandemic this was only exceptionally the case, but in the less contagious after epidemics it was more frequent.
In the following examination of the distribution of influenza in closed institutions, upon ships, and in hospitals, we shall frequently meet with noteworthy exceptions to the rule-that is to say, we shall find many instances of immunity from influenza, in spite of the implantation of the same within narrow confines. We must, therefore, once for all emphasize the fact that in this respect influenza acts the same as any other contagious infectious disease, like diphtheria, scarlet fever, measles, etc. These exceptions to the rule surprise us in the case of influenza only because we think of this disease as a highly contagious pandemic affection. But not every one who is affected by influenza distributes its contagion, and not every one disseminates fully virulent germs. Cases characterized by exclusively nervous or gastro intestinal symptoms, without secretion products from the respiratory mucous membrane, are very likely harmless. Another case may disseminate germs which perchance do not come in contact with the surrounding individuals. Chance acts as a very important factor in the distribution of contagious diseases.
But in spite of exceptions the rule remains that the patient gives off virulent germs in such numbers that his whole family, his household, and all who come in contact with him are immediately infected.
The literature, especially of the last pandemic, is replete with examples which prove the importation of the disease from an infected area into one which had not been infected. Small towns, villages, hamlets, farms, passes, ships, and inclosed institutions furnish the most cogent proofs.
On innumerable occasions it has been shown that an individual returning from afar to his own village or farm, itself far removed from external communication, had imparted the disease, at a time when for far and wide around there was no influenza. After the infection of the members of the family of this individual, the tenants of the house contracted the disease, and the subsequent development of the pest could be easily traced, in the smaller places, as radiating from this original focus. The English collective investigations of 1782 already called attention to such cases of importation and infection from influenza. We will mention a few examples:
A good field for observation was London, to which daily crowds of persons travel by rail in order to attend their business, returning again in the evening to their houses and villas. With an unusual monotony the following sentences occur again and again in the English Collective Investigation, namely: " The first case of influenza was a man who went to London daily," or " All the earliest cases were men going to London daily, their wives and families being affected later." The weekly markets and fairs in England played an important part in the dissemination of the pestilence from city to country. More rarely than one would suppose did the first cases of influenza in a place begin in a hotel (Lubeck, Strass burg, Davos).
A physician leaving Berlin on the seventh of December was taken ill at his home, Elgersburg (Thuringen), with influenza, on December 8, but, notwithstanding, made several visits in his native town. A few days thereafter those visited by him were taken ill, Elgersburg and its vicinity up to that time not having had a single case of influenza (P. Friedrich,. loc. cit.).
On January 4, 1890, an owner of a slate quarry returned to his home, Llanegryn, ill with influenza, from London, where he had lived in a house infected by the disease. On January 6 nine other inmates of the house where he had lived in London, and five to six days thereafter the workmen of the slate quarry, were taken ill (Parsons' report, loc. ext.).
In quite a number of cases Pribram could accurately trace how persons ill with influenza coming from Vienna, in which city the disease was prevalent, to the city of Prague, which was still exempt, infected their relatives and other inmates of the houses where they lived.