Leichtenstern believed that the usual incubation period is from one to three days although some cases have been reported in which it is without doubt no longer than twelve hours. Parsons in reporting for England also gives the incubation period as from one to three days as a rule.
It is reported in France in 1918 that in one institution thirty-one cases out of thirty-three individuals occurred within three days, all of them infected by one nurse.
MacDonald and Lyth report in the British Medical Journal for November, 1918, an interesting observation concerning the incubation period in influenza. These two individuals were traveling from London to York in the same compartment with an individual who was just convalescing from influenza. Exactly forty-one hours after being on the train with this individual, they both came down with the disease. One suffered lightly while the other was severely ill. The wife and two children of the latter contracted the disease in turn, and with them also the first symptoms appeared suddenly after a delay of about forty-eight hours.
Stanley, in studying the epidemics of influenza in San Quentin Prison, found that as a rule there was an increase in incidence following the Sunday picture shows. This usually occurred on Tuesdays and Wednesdays, giving an apparent incubation period of from thirty-six to sixty hours. He tabulated the records of twenty-nine individuals who had presumably become exposed at the show and found that the incubation period averaged about forty-eight hours.
The majority of observers give the incubation period as from twenty-four hours to four or five days, most often two or three days.
Predisposing Causes.
Not every individual acquires influenza. There are those who assume that the disease is so wide spread that every individual in each community attacked has been actually exposed to the disease. In that case there must be a certain amount of natural immunity which protects around sixty to eighty per cent. of most populations from the disease. The other extreme would be that every exposed individual falls victim to the disease and that only twenty to thirty or forty per cent. are actually exposed. The true state probably lies between these two extremes.
Nevertheless it is a fact that some individuals naturally insusceptible to the disease fall victim as a result of the action of some extraneous force, something which lowers their resistance. Raw recruits in the army camps in the fall of 1918 contracted the disease in much greater proportion than did the hardened soldiers. Fatigue, intercurrent illness, environmental changes and exposure to inclement weather may all predispose to infection in the individual. Greenwood found that the compulsory rationing of food in England during the war was probably not a predisposing cause of infection. The incidence of the disease in the South Africa Union where food was abundant was even higher than that for the British Isles. Hamer calls attention to the fact that the ages of highest incidence during the pandemic were those ages in which the diet was perhaps more restricted than in other ages. This, however, is but one factor and cannot be accepted as conclusive.
It had been suggested that in the army camps in the United States typhoid vaccination during the epidemic predisposed to the disease. The similarity of the symptoms in vaccine reaction and in influenza may have suggested this. V. C. Vaughan has investigated this possibility and finds that those organizations in which anti-typhoid vaccine was discontinued for a time after the appearance of the influenza suffered quite as severely as those which submitted to vaccination.
Other predisposing causes, such as the incidence of crowding in a household and the sanitary surroundings of the individual will be discussed later.