[Illustration: Deaths from tuberculosis compared with other contagious diseases in the city of New York in 1908.] Tuberculosis.--The one disease responsible for the greatest number of deaths--perhaps one seventh of the total on the globe--is tuberculosis. It is estimated that of all people alive in the United States to-day, 5,000,000 will die of this disease. But this disease is slowly but surely being overcome. It is believed that within perhaps one hundred years, with the aid of good laws and sanitary living, it will be almost extinct. [Illustration: This curve shows a decreasing death rate from tuberculosis. Explain.] Tuberculosis is caused by the growth of bacteria, called the _tubercle bacilli_, within the lungs or other tissues of the human body. Here they form little tubers full of germs, which close up the delicate air passages in the lungs, while in other tissues they give rise to hip-joint disease, scrofula, lupus, and other diseases, depending on the part of the body they attack. Tuberculosis may be contracted by taking the bacteria into the throat or lungs or possibly by eating meat or drinking milk from tubercular cattle. Especially is it communicated from a consumptive to a well person by kissing, by drinking or eating from the same cup or plate, using the same towels, or in coming in direct contact with the person having the germs in his body. Although there are always some of the germs in the air of an ordinary city street, and though we may take some of these germs into our bodies at any time, yet the bacteria seem able to gain a foothold only under certain conditions. It is only when the tissues are in a worn-out condition, when we are "run down," as we say, that the parasite may obtain a foothold in the lungs. Even if the disease gets a foothold, it is quite possible to cure it if it is taken in time. The germ of tuberculosis is killed by exposure to bright sunlight and fresh air. Thus the course of the disease may be arrested, and a permanent cure brought about, by a life in the open air, the patient sleeping out of doors, taking plenty of nourishing food and very little exercise. See also Chapter XXIV. [Illustration: This figure shows how sewage from a cesspool (_c_) might get into the water supply: _lm_, layer of rock; _w_, wash water.]