Sunlight is more positive than the wind, and the exposure to direct sunlight of a bottle filled with disease germs will kill them all in two or three hours. The surface layers of a pond never have as 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:—