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and especially in the sputum of those suffering from pulmonary
tuberculosis. No discharge is effective unless it contains the
tubercle bacillus.
3. Cases of tuberculosis of bones, glands, and internal
organs from which there is no discharge or which do not furnish
any excretion, and cases of arrested pulmonary tuberculosis,
have never been proved to be infectious.
(By arrest is here meant that all the symptoms and
physical signs of activity have disappeared, and the sputum has
either ceased or no longer contains tubercle bacilli).
4. The means by which tubercle bacilli may enter the body
are:—
(a) By inoculation through a wound or abrasion of the skin.
This has occasionally occurred to workers in laboratories,
post-mortem attendants and others dealing with tuberculosis
material, and is presumably the way in which lupus is
acquired.
(b) By inhalation. Susceptible animals are readily
infected by the inhalation of air containing tubercle bacilli,
whether in droplets or suspended as fine dust, but in the spread
of the disease among human beings the latter appears to be
the more important means of infection. The sputum or other
discharges, whether on soiled handkerchiefs, linen, garments, or
elsewhere, when dried, may become pulverised, and in this
condition may be readily dispersed in the air of a room. That
droplets of sputum are less important agents of infection is
suggested by the fact that the incidence of consumption upon
the staff, nurses, and others engaged in hospitals for the
treatment of tuberculous disease, where all discharges are
carefully disposed of, is not above the average in the general
population.
(c) By swallowing. Dust infected by the tubercle bacillus
may be conveyed to food and so enter the alimentary canal;
or infection may occur more directly in the act of kissing, or by
consumptive and healthy persons using the same food utensils.
As about 10 per cent. of the milk supplied to large cities
contains tubercle bacilli derived from infected cows, this avenue
of infection is particularly important in the case of children.
The bovine tubercle bacillus is more commonly responsible for
tuberculosis in young children than in adults, but the proportion
of cases due to it varies very much in different localities.
There is no evidence that tuberculosis can be conveyed to
others either by the breath alone, or by emanations from
patients, or by their garments, unless soiled by dried sputum
or discharges.
5. The spread of tuberculosis is favoured by uncleanliness,
overcrowding, and imperfect ventilation, and is hindered by
the opposite conditions. Experience in hospitals and other