Manner of Spread.
More characteristic and more important from an epidemiologic standpoint than the symptomatology in general, as we have discussed it, is the mode of development of the epidemic as a whole.
Human intercourse.—Before the days of bacteriology the contagiousness of the disease was little discussed. Its infectiveness was in fact not universally established until the epidemic of 1889–1890. One of the first writers who attempted to see in the influenza a contagious disease was Ch. Calenus who wrote in 1579: “Contagiosum dico morbum, quia etsi quidem ab occulta quadam coeli influentia, principaliter eum profisci haud dubium est ... eo in loco quo jam grassabatur inter homines citius eos invadabat, qui cum affectis frequenter conversabantur, quam eos, qui a consuetudine affectorum studiose abstinebant.” This keen observer saw that those who carelessly exposed themselves to close contact with cases of influenza were more likely to develop the disease than those who protected themselves in every way possible. The “contagious” school first developed in England, where Haygarth, Hamilton, Gray, Hull, Duggard, Bardsley, and others, in 1775–1803 described the disease as being not in the air, but in a specific contagion. Others who considered influenza a contagious disease were Simonin, Lombard, Petit de Corbeil (1837), Blanc (1860), and Bertholle (1876).
Watson (1847) quotes Cullen as saying that this species of catarrh proceeds from contagion. He, himself, is not convinced of this fact. He says the visitation is too sudden and too widely spread to be capable of explanation in that way. “There are facts in the history of influenza which furnish a strong presumption that the exciting cause of the disorder is material, not a mere quality of the atmosphere; and that it is at least portable. The instances are very numerous, too numerous to be attributed to mere chance, in which the complaint has first broken out in those particular houses of a town at which travelers have recently arrived from infected places.... What I wish to point out now is the fact that the influenza pervades large tracts of country in a manner much too sudden and simultaneous to be consistent with the notion that its prevalence depends exclusively upon any contagious properties that it may possess.”
Parkes, writing in Reynolds’ System of Medicine in 1876, views the subject more as we see it today: “The rapidity of the spread would seem at once to negative any connection between human intercourse and the propagation of the disease; yet there is some affirmative evidence. It does not appear to follow the great lines of commerce; but when it has entered towns and villages in which the investigation can be carried on, it is curious how frequently the first cases have been introduced, and how often the townspeople nearest the invalids have been first affected. In this country especially, Haygarth in 1775 and 1782, and Falconer in 1802, collected so many instances of this that they became convinced that its propagation was due entirely to human intercourse. So also, when it passes through a house, it occasionally attacks one person after another. But if it is introduced in this way it afterwards develops with marvelous rapidity, for we cannot discredit the accounts of many thousands of persons being attacked within a day or two, which is quite different from the comparatively slow spread of the contagious diseases. This sudden invasion of a community makes it, to many persons, appear highly improbable that any effluvia passing off from the sick should thus so rapidly contaminate the atmosphere of a whole town.
“Still, we must remember how singularly, of late years, the knowledge of the introduction of cholera by persons coming from infected districts has increased, and how very striking are the instances of this kind already recorded in several works on influenza.
“In some cases, again, isolation or seclusion of a community, as in prisons, has given immunity; or at least that community has not been attacked.”
The great rapidity of spread has caused even in 1918 some temporary doubt as to the contagiousness of the disease. Thus, Zinsser wrote:
“The opinion of direct and indirect transmission from man to man is also well supported by a detailed study of the epidemiology of individual outbreaks. In our own experience with epidemics such as those at Chaumont, Baccarat and other places, the suddenness with which the malady attacked large numbers of people at almost one and the same time, caused me at first to be exceedingly skeptical of accepting transmission by contact as the only means of conveyance. We considered food and insect transmission as possibilities, and tried our best to find grounds for involving such agencies. But in every case we were forced to return to the conclusion that direct and indirect contact between men came nearest to doing justice to all observed facts.”
There have been many examples reported from personal experience to show that influenza is transmitted from man to man. Two objections, however, have had to be met, before this view was generally accepted. First, it has been claimed by some that the disease spread more rapidly from an assumed focus than individuals could travel, and second, that instances were on record of cases occurring spontaneously in isolated communities. Yet a third argument formerly raised against the contagious character of the disease was the claim that it broke out in mass attacks, that large numbers became ill on the same day without the occurrence of isolated antecedent cases. The splendid work of epidemiologists following the 1889 epidemic appears to have answered all of these objections. Many, such as Leichtenstern, have gone into great detail on this subject. In fact, at that time this was the question of greatest importance. Today we assume the correctness of the hypothesis, and pass on to consideration of other subjects of more recent development. We will, therefore, review very hurriedly some of the evidence quoted to prove that influenza is transmitted only from man to man and only by human intercourse.