Predisposing Causes.— Though the bacillus of tuberculosis is the actual cause of the disease whenever it occurs, other factors require con sideration. As wheat will not grow on every soil, so the tubercle-bacillus will not grow in every individual. In fact it would appear that the majority of human beings are quite insus ceptible, and that as a rule an overwhelming dose of the organisms repeated frequently for some time is necessary to overcome the resist ance. All the conditions necessary to produce susceptibility arc unknown, yet it is empirically true (and could be with reason supposed) that any circumstance which tends to lower the gen eral vital resistance decreases the resistance to tuberculosis. Therefore, defective and insuffi cient food, over-work, worry, chronic alcohol ism, surroundings like a damp, dark, over crowded dwelling, persistent irritation of a somewhat naturally susceptible part of the body, as irritation of the lungs by the constant in halation of dust (mine-workers, stone-cutters, etc.), previous severe disease like typhoid fever, etc., all tend to increase the susceptibility.
It was thought in the past that the most common cause active in the production of sus ceptibility was heredity, because the disease manifests itself more commonly in the children of the tuberculous than of the non-tuberculous. Recognizing the communicability of the disease, however, the closeness of the contact after birth easily accounts for this, without it proving them more susceptible. In fact, a strong argument can be deduced to the contrary, namely, that the children of tuberculous parents are less sus ceptible. Granting the communicability, it is not a surprise that children who are kissed and fondled for years by tuberculous mothers con tract it, but it is a surprise if any escape; and if they were more susceptible we would expect them to contract it in such a virulent form that no child of a tuberculous mother would ever reach adult age. The fact is, however, that the majority of children of tuberculous parents never manifest the disease and the ones who do usually manifest it in a very chronic fashion and only after the age of 15. In other words, though these children live in an atmosphere impregnated with the germs, the majority fail to contract it and the remainder resist it for years.
Flick's paper on tuberculosis as a house disease goes to prove its communicability. He investigated all the houses of the largest, oldest and most thickly populated ward in Philadel phia, and found that the deaths from tuber culosis in that ward were disproportionately large in certain houses. In short, he demon strated case after case of apparently healthy families moving into a house previously oc cupied by a tuberculous person with the result that one or more members died of the disease.
Modes of Infection.— There are four pos sible modes of infection, namely, inoculation, heredity, inhalation and ingestion.
Inoculation.— Villemin's work, supple mented by that of Cohnheim and Salamonson, absolutely established the fact that the disease was inoculable. Inoculation is, however, quite rare as a method of general infection in human beings, and its occurrence is practically limited to special occupations. Inoculation with the production of a strictly limited local lesion is reasonably common on the hands of physicians who do anatomical or post-mortem work (the post-mortem wart, the (eichen-tubercle of the Germans), of butchers, tanners, etc. Local tubercles have also been produced by piercing the• ears for earrings, by tattooing and by washing the clothes of a tuberculous patient. Both local and generalized tuberculosis have been reported as a result of the rite of cir cumcision, the last step in which is the sucking of the wound.
Heredity.— Up to the time of Villemin this was the generally accepted mode of acquiring the disease, though here and there down the centuries from the time of Galen some one has stood out against it in favor of contagion. Hereditary transmission has been experi mentally proven on the lower animals (Gart ner), and occasionally demonstrated in human beings by the finding of tuberculous lesions in the foetus. These proofs occur so rarely, however, that the ordinary view of practically all cases being examples of contagion, is fully warranted.
Inhalation.—The common belief at the pres ent day is that the majority of cases of tubercu losis are the result of inhalation of the germs. The contagiousness of the disease being pi-alien, and the infectious bacilli being found in the matter given off from a tuberculous ulcer (therefore, in the sputum in tuberculosis of the lungs), it is readily understood how people liv ing with a consumptive may be more or less constantly inhaling the contagion. Nuttall's estimate of the number of bacilli thrown off in the expectoration daily could only make one wonder how anybody escapes the disease, were it not that they are so easily and quickly devital ized. In a case where the patient expectorated• about four ounces daily, Nuttall estimated the number of bacilli to be from one and a half to four billions in the 24 hours. Experiments on animals with the dust of rooms occupied by tuberculous patients have usually proven posi tive (Cornet). The arguments for inhalation as the most common mode of infection are: (1) the very great frequency of tuberculosis of the lungs; (2) the frequency with which all per sons are exposed to this form of contagion.