many bacteria
in them as the lower layers, partly on account of the sedimentation, but
largely because they are killed by the direct action of sunlight. The
bacillus of consumption and bacillus of diphtheria are both killed in an
hour or so by direct sunlight. This is one reason why living rooms
should have sunny exposure and why, on the other hand, disease thrives
in dark tenements.

The soil is the third natural method of disinfection, not because the
soil itself destroys bacteria, but because in the soil are to be found
millions of non-harmful germs and these germs are hostile to the
disease-producing germs, so that they destroy their virulence. It is on
this principle that the wastes from typhoid fever patients are buried in
the garden, the presumption being that the bacteria there present will
destroy the typhoid fever germs before they can escape and do any harm.
While this action undoubtedly exists, it is not positive enough to
depend upon, and disinfection by the use of chemicals should always be
practiced.

CHAPTER XVI

_TUBERCULOSIS AND PNEUMONIA_

These two common widespread diseases affecting the lungs may be
discussed together, although they are not closely related in origin or
effects.

_Tuberculosis._

That form of tuberculosis known as consumption is at present the most
prevalent and the most dreaded of all infectious diseases. In 1908, in
the Registration Area of the United States (about one half of the whole
country), it caused 67,376 deaths. Deaths from other infectious diseases
are shown in the following table, together with the population:--

TABLE XVIII. SHOWING DEATHS FROM VARIOUS INFECTIOUS DISEASES IN THE
UNITED STATES, 1908

Population of Registration Area 45,028,767
Deaths in Registration Area 691,574
Deaths from tuberculosis 67,376
Deaths from pneumonia 61,259
Deaths from diarrhoea (chiefly of babies) 52,213
Deaths from cancer 33,465
Deaths from typhoid fever 11,375
Deaths from diphtheria and croup 10,052
Deaths from scarlet fever 5,577
Deaths from whooping cough 4,969
Deaths from measles 4,611
Deaths from smallpox 92
Deaths from hydrophobia 82
Deaths from leprosy