One attack of influenza in the majority of cases confers protection from the disease for some time, but the degree of immunity does not approach that conferred by small pox, whooping cough, scarlet fever, measles, or enteric fever. Certainly, during the short, eight week epidemic of 1889-1890, a second attack was a rare occurrence. But from the later epidemics it became evident that this immunity was merely a question of time. Cases of recurring attacks of the disease occurred in one and the same individual, and even in the same family at different times, and were common enough in the last influenza period. Some authorities greatly overestimate this immunity. For instance, Baumler thinks that the aged, who in the last pandemic showed such extraordinarily slight morbidity, still retained the immunity conferred upon them by passing through an attack of influenza in the epidemic years of 1837, 1847, and 1857. Regarding this, Ed. Gray mentions already in 1782 that " it was also remarked that many persons who escaped the epidemic of 1775 were affected by that of 1782, and many who escaped the latter were affected by the former " (E. S. Thompson, p. 110).
Several physicians, especially many English and French, as Teissier, Squire, Joy, and others* believe that one attack of influenza actually predisposes to further attacks. A. Gottstein considers this increased predisposition to be an explanation of influenza becoming endemic, whereas we believe that the growing immunity of the population accounts for the successive decrease in the morbidity of the later epidemics. (See p. 549.)
* See Parson's report.
This apparent contradiction becomes intelligible when we distinguish clearly between relapses and new infections. Relapses are the rule in influenza. "La grippe est une maladie arechutes." Relapses occur frequently even after the patient has been up and about or has left the house for the first time, and are usually ushered in by some complication or sequela. No doubt many writers have erroneously termed these late relapses new infections, and have based upon them their opinion of "increased predisposition."
On the other hand, those cases should not be counted as relapses in which a convalescent influenza patient, after two or more weeks, has a rigor and passes through a second typical attack of influenza. The attempt to discard such unquestionable reinfections by the supposition that the first or the second attack might have been an ordinary coryza, is an explanation which cannot be maintained against the actual facts.
The question asked in the German collective investigations, " How often have you seen the relapses? " was in itself so ambiguous that a direct answer could not be expected. Of the observers, 10 per cent, saw no relapses, 63 per cent, saw them but rarely, and 23 per cent, frequently.
Of greater value are the following statistics: Turney, at St. Thomas's Hospital in London, found that 5 per cent, of the influenza patients admitted in the spring of 1891 had had influenza during 1889-90. Dickson found that those custom and postal officials of London who had had influenza during 1889-1890 furnished but half as large a contingent in the second epidemic as those who had remained unattacked in the first epidemic (5 to 10 per cent.). In the epidemic of 1891-1892, of 272 influenza patients admitted to the Urban Hospital (Berlin), 8 per cent, had had influenza during 1889-1890; of 105 pupils in the seminary at Preisketschan, 32 per cent.; while of 122 influenza patients in the seminary at Walden burg (Saxony), 35 per cent.; of 905 patients in Liibeck, 24 per cent., and of the private patients of Caldwell-Smith, 25 per cent., had been attacked by the disease during the epidemic of 1889-1890. In the industrial school at Swinton (Manchester), which was cut off from communication, 171 children out of 589 had influenza during the first epidemic. Of these 171 children, 2.6 per cent, became affected for the second time in 1891, while of those who had remained exempt in the first epidemic, 5.7 per cent, contracted influenza during the second epidemic.
In addition to the acquired immunity there is also a congenital or natural one. Infants to some extent possess such an immunity. But many adults, for instance, physicians and nurses, who were in constant contact with influenza patients, remained exempt from the disease during all the epidemics. In other cases this form of immunity is only a temporary one. Many physicians who escaped during the first epidemic had to pay their tribute to the disease during later epidemics.
At the beginning of the pandemic of 1889, by reason of the sudden affection of the masses, the assertion was often made that influenza had no period of incubation; that we had to deal with a miasmatically distributed toxic substance, which, immediately on entering the human organism, displayed its morbific character. At the present time we need mention this theory only to reject it, although it was then held by many leading medical authorities.
No doubt the time elapsing between the inception of the germs, viz., the moment of infection and the moment at which symptoms appear, is short-in the greater number of instances it is from one to three days. But numerous observations are recorded, even from the earlier epidemics, which make it probable that the period of incubation may be as short as twelve hours. The duration of the period of incubation, according to the English collective investigation, may be taken, from the concordant answers of the majority of observers, to be from one to three days (Parsons' reports).