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An analysis of occupational mortality statistics brings out the
conditions of labour associated with high mortality. Pulmonary
phthisis as a cause of death may be said to stand on a tripod of bad
feeding, bad housing, and bad habits. No doubt part of the fall of the
death-rate may be due to a lessening of all three of these, but improved
hygiene is not in itself sufficient to account for the fall any more than
the segregation in institutions. Surely under the conditions of town
living more fully urbanized populations demand and receive a greater
amount of institutional treatment. Institutional treatment is merely
an incident of urbanization. As a means of preventing infection,
hygiene and institutional treatment alike seem to fail. It is indeed
difficult to see how it is possible to prevent infection in the case of
tuberculosis. We are all acquainted with cases of consumption that
do not seem to progress either to recovery or to death. There must be
an enormous number in the population at any time who are continually
expectorating infectious sputum, and who are at the same time scarcely
cognizant of their disease. Although discharging tubercle bacilli constantly,
they are only comparable as regards their general health to
cases of latent tuberculosis in which no such discharge is occurring.
Too much attention has been paid during the last thirty years to infection
by a visible, easily recognizable micro-organism; too little
attention has been paid to the invisible, and not easily recognizable,
resistance of individuals. Our great difficulty is to say how far infection
by tubercle is capable of producing an acquired immunity to
tuberculosis. We can say with some certainty that there are environments,
and conditions of race and heredity, which render people
particularly susceptible.
It is very difficult to say of any individual life that it has become
immunized because of infection. We can infer in a great number of
individuals that when infected they are resistant, and that if placed
under suitable conditions they will recover.
Various attempts have been made to classify cases of pulmonary
phthisis, at first by physical signs, then by a combination of physical
signs and constitutional symptoms, and still more recently by constitutional
symptoms alone. Under the last classification we endeavour
to group our cases according to the manner in which they seem to
stand infection. We endeavour to measure their immunizing ability.
By a ''careful selection"of cases in which a high immunizing ability
can be inferred, we are able to promise a high rate of recovery. It is
noteworthy that "carefully-selecteds" cases, by whatever means they