5. Experience shows that dusty occupations, indoor occupations, alcoholism, over-fatigue, an attack of acute illness, especially of influenza, measles, or enteric fever, increase the danger of minimal doses of tubercle bacilli, and serve to bring latent foci of disease into activity.
Explanations of the Decreasing Death-rate from Tuberculosis
In the light of the above facts, how is the steady and continuous decline in the death-rate from tuberculosis during the last fifty years to be explained?
(a) No support is given by animal experiment to the assumption that the types of human bacillus infecting mankind have declined in virulence; and changes in the severity of consumption historically or currently in different races of mankind are equally explicable on the ground of differences in social misery, in sanitary conditions and associated heavier dosage of infection and neglect of treatment.
(b) The facts do not appear to me to be reconcilable with the assumption that natural selection has increased human resistance to infection by tuberculosis; though, were this so, it would not justify refraining from every possible effort to control infection and to treat every tuberculous patient by the best known methods. Tuberculosis is an ancient disease, there being evidence of it in Egyptian mummies 1000 years B.C.; and any selective agency has, therefore, had ages for its operation. If the steady decline—approximating 2 per cent. per annum in the death-rate from pulmonary tuberculosis in England during the last thirty or forty years—has resulted from the acquirement of racial immunity, it is remarkable that a somewhat similar decline has occurred almost simultaneously during the last forty years in Great Britain, Germany, and America; while in France, Norway, and Ireland there has been little if any decline, or it has occurred only in very recent years.
To assume that susceptibility to the tubercle bacillus in the course of its natural history has diminished in England, and that Ireland has not shared in this privilege would be to add one more to Irish grievances! This assumption does not fit in with international facts; which point rather to the conclusion that, during the period in question, unsatisfactory sanitary and social circumstances, including opportunities for massive and protracted infection, have continued to a greater extent and for a longer time in Ireland and France than in Great Britain, America and Germany.
(c) If the assumption of increasing racial immunity does not consist well with all the facts, more perhaps can be said in favour of the unproved hypothesis that a high proportion of the population are from time to time temporarily immunized by small doses of tubercle bacilli; and their resistance to larger doses of infection thereby increased. Experimentally calves inoculated with small doses of tubercle bacilli remain during the next year or two unaffected by much larger doses of tubercle bacilli, unlike calves not submitted to this treatment. Tubercle bacilli are somewhat widely distributed, though they occur chiefly in the immediate environment of careless consumptive patients; and it is conceivable that minimal doses of bacilli may arouse the resistance of the cells and fluids of the body and prepare them to resist successfully larger doses of infection. This is consistent with the fact that while one in about ten deaths from all causes is caused by tuberculosis, a majority of the total population are shown by pathological evidence to have been at one time or another infected by tuberculosis, and yet have either never been ill, or have recovered, usually without the existence of tuberculosis being detected or even suspected. Obviously this is satisfactory evidence that mankind is relatively resistant to the infection of tuberculosis.
The fact just mentioned naturally leads to the question: what determines the result when tubercle bacilli invade the human subject? Assuming fairly uniform virulence of tubercle bacilli, the result for an infected person depends on two factors: the dosage of infection, and the resistance of the cells and fluids of the invaded person; and evidently increase in the dosage of infection and lowering of personal resistance may have identical effect in determining serious disease. Of the importance of the already mentioned factors which lower personal resistance to disease,—often also at the same time increasing infection,—there can be no doubt.
It is impossible in most instances to set out separately circumstances increasing infection from circumstances lowering resistance. During the last three or four decades there has been improvement in respect of the factors lowering resistance to attack, but there has been simultaneously a great decline in opportunities for infection on a massive scale, as a result of habits of greater cleanliness, especially in regard to spitting, of diminished overcrowding of population, and of increased treatment and the incidental segregation of advanced cases of disease in hospital beds.