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which kills in the middle and later stages of life, has its greatest mortality at the age periods of middle
life when many of its victims are not only in the period of their greatest activity, mental and physical,
but also are most likely to have others (husband or wife, and children) dependent upon them. There
is the further important consideration that its disabling influence may, in consequence of the long period
of the illness, be the means of gradually dragging the victim, and those dependent upon him, steadily
down in the world owing to inability to earn a decent livelihood, and consequent upon th.s there often
results many a heart.breaking domestic tragedy of terrible hardship, misery, poverty and broken up
homes, not forgetting the increased opportunities arising for the spread of infection to others and the
repetition of the whole ghastly cycle of event. The disease, therefore, cannot be regarded as anything else
but a great social evil.
The decline in the death rate from tuberculosis, up to at all events very recent years, is one of
the most remarkable phenomena in Vital Statistics. It is so remarkable that some authorities are not
satisfied even now that the decline is a real one. The Statistics are, of course, based upon death
certificates and it is common knowledge that these are often based on faulty diagnosis, and probably
such faulty diagnosis was more frequent in past years. The general opinion, however, of many expert
statisticians who have carefully investigated the official returns of the last 50 years, is that they may
be reasonably regarded as an index of the variation, in the death rate from this disease.
We may assume, therefore, that apparently there has been a considerable decline in the death
rate from tuberculosis during this period, or, to put it in another way, the proportion of people who die
of tuberculosis is now considerably less than it used to be. Formerly one death in eight was due to
tuberculosis, now apparently it is about one in ten. That most interesting, important and very
practical question therefore now arises—
What are the causes of the decline?.—On looking at the chart (E. & W.) (Appendix II.) it
will be noticed that until quite recent years the decline has been almost a uniform one. Slight undulations
occur every ten years, or thereabouts, but no marked change in the rate of decline occurs anywhere
indicating any particular event associated with any special anti.tuberculosis measures, such as the growth
of the sanatorium movement or the introduction of tuberculin as a method of treatment. The causes of
the decline then, whatever they may be, have evidently been acting more or less continuously during
the whole period—What are they ?—If it were possible to supply the correct answer to this supremely
important question, no great difficulty would be experienced in deciding upon the exact nature, extent
and cost of the various measures necessary for controlling and eventually possibly extinguishing the
disease.
Before proceeding to give a brief account of some of the theories which have been advanced to
explain the decline of the death rate from tuberculosis in England and Wales up to quite recent years,
attention should be drawn to the very different behaviour of the death rate from tuberculosis in Ireland
(Appendix II.).
Explanations of the Decline of the Death Rate from Tuberculosis.
(1) The tubercle bacillus is undergoing a slowly progressive diminution of virulence.—There is
nothing inherently improbable in this theory, because in the case of scarlet fever there appears at all
events to be some ground for thinking that the virulence of the causative organism, for whatever reason
it may be, has notably diminished. The mortality of the disease to.day is only about one.tenth of what
it was fifty years ago, yet scarlet fever is just as widely prevalent as ever. It is possible that this wide
prevalence of the disease has brought about an immunisation of the race by a gradual elimination of the
stocks most prone to the disease.
There is no positive evidence to show that the tubercle bacillus has lost any of its virulence and
the fact that the disease has behaved in a totally different way in Ireland is very much against the theory
of diminished virulence of the bacillus. Furthermore, the experience of the Royal Commission on
Tuberculosis went to show that the stability of the virulence of the bacillus of tubercle was a remarkably
characteristic feature.
(2) The intrinsic capacity for resistance of the race is increasing.—There is more to be said for the
view that the fundamental power of resistance to invasion by the tubercle bacillus has been increased
among peoples who have long been subjected to tuberculosis by the slow but continuous elimination of
the more susceptible families. This view assumes that high or low powers of resistance to the disease
are transmitted by inheritance, a view which, although generally held (and equally applicable to the analogy
of scarlet fever referred to above), cannot be said to have been proved. Nevertheless, there are many
strong reasons for believing that race immunisation is a powerful factor in connection with death rate.
In this connection it is worthy of note that certain races show a very high degree of susceptibility to
tuberculosis (1) when they change their residence or (2) when persons suffering from tuberculosis come to
live amongst them. As instances of the first—Metchnikoff cites the ravages of tuberculosis amongst
Kalmuck students from Turkestan who came to be educated in Russia, and more recently during the
progress of the war there have been some striking evidences of a similar kind recorded by French
observers amongst native Troops (Senegalese) brought to serve on the Western Front. As illustrations
of the second there are several recorded cases of the virulence of the disease when it was first introduced
amongst South Sea Islanders, North American Indians and natives of South Africa, by cases of tubercu
losis going to these countries from this and other countries where the disease was widely prevalent.
Admitting, however, that there has been some immunity established amongst nations like those
where the disease has been prevalent for many centuries, yet it is clear that the process must have been
a very gradual one, and obviously this theory cannot possibly account for the behaviour of the death rate
either in England or in Ireland during the last half century.