The question of the contagiousness of influenza is one of grave interest, and has been the subject of much controversy. The great rapidity of the spread of epidemics, the vast area they overrun, the fact that they do not follow the lines of human intercourse, the suddenness with which great numbers of the inhabitants of an invaded district or city are seized, the fact that the most complete seclusion from intercourse with affected persons, or even the shutting up of houses, affords in most instances no protection whatever,—all go to show that the disease spreads, in the main, independently of direct contact. This opinion has been almost universally entertained. There is evidence, however, to show that the disease is to some extent contagious; and so convincing have the facts bearing upon this point appeared to some that they have believed it to be propagated entirely by human intercourse. Haygarth16 declares, as the result of his observations during the epidemics of 1775 and 1782, that the influenza spreads "by the contagion of patients in the distemper;" and Falconer,17 writing of the epidemic of 1803, says, "I have no doubt that it is contagious in the strictest sense of the word." Watson18 regards the instances in which the complaint has first broken out in those particular houses of a town at which travellers have arrived from infected places as too numerous to be attributed to mere chance. Very often those dwelling near the invalids are attacked next in the order of time, and when the disease affects a household all do not usually manifest the symptoms at the same time, but one member after another is stricken down with it.
16 John Haygarth, M.D., F.R.S., On the Manner in which the Influenza of 1775 and 1782 spread by Contagion in Chester and its Neighborhood.
17 William Falconer, M.D., F.R.S., An Account of the Epidemic Catarrhal Fever, commonly called the Influenza, as it appeared at Bath in the Winter and Spring of the Year 1803, Bath, 1803.
18 Principles and Practice of Medicine.
In a few rare cases the isolation or seclusion of a community has appeared to give protection, as in cloisters, prisons, garrisons, and the like; at all events, there are instances on record where segregated communities of this kind have escaped attack.
The following observation, conducted under unusual circumstances, establishes the fact that influenza may be brought from an infected city in such a way as to give rise to a localized outbreak in a remote community. Drs. Guitéras and White19 narrate that, influenza prevailing in Europe, and particularly in Paris and London, an American gentleman in bad health contracted the disease in London, improved, suffered a relapse shortly afterward in Paris, and died there at the end of December, 1879. His body was embalmed and sent home. Following the exposure of the remains of this person to the view of his family in Philadelphia there was an outbreak of influenza with characteristic symptoms, which affected, in the first place, members of that family; afterward, friends living in close intercourse with them; next, the medical attendant of some of them; and finally, the housekeeper and a patient or two of one of the physicians who wrote the paper, the whole number affected in Philadelphia being eighteen at the time of the publication of the account. Subsequently two or three other cases were developed, but the disease did not extend beyond the immediate circle of those in direct communication with the invalids.
19 John Guitéras, M.D., and J. W. White, M.D., "A Contribution to the History of Influenza, being a Study of a Series of Cases," Philadelphia Medical Times, April 10, 1880.
It was at one time thought that influenza developed at once, without a period of incubation, persons in perfect health being struck down with it as by lightning-stroke. It is, however, now known that a period of incubation, varying from a few hours to several days, and usually without subjective symptoms, exists. Many instances are recorded in which persons coming into an infected city have remained well for one, two, or three days, but have eventually shared the sufferings of those into whose midst they have come. There are cases also in which the period of incubation could not have been less than two or three weeks.
There is no sufficient evidence of a causal relation between influenza and any other epidemic disease. The statement that other prevalent diseases abate in frequency and intensity upon its outbreak is not sustained by well-observed facts. Graves20 holds that those suffering with acute diseases are less liable during the febrile stage, but that they are attacked as convalescence sets in.
20 Clinical Medicine.