THE DIRECT CAUSE.--The direct cause of tuberculosis is Koch's _Bacillus tuberculosis_. This is a slender, rod-shaped microorganisms (Fig. 88) occurring in the diseased tissues, feces and milk of a tubercular animal. It belongs to that small group known as acid-fast bacteria. The tubercle bacillus is not really destroyed by external influences, and it may retain its virulence for several months in dried sputum if protected from the light. Its vitality enables it to resist high temperatures, changes in temperature, drying and putrefaction to a, greater degree than most non-spore-producing germs. Direct sunlight destroys the germ within a few hours, but it may live in poorly lighted, filthy stables for months. A temperature of 65\260 C. destroys it in a few minutes. Animals that, have advanced or open tuberculosis may disseminate the germ of the disease in the discharge from the mouth, nostrils, genital organs, in the intestinal excreta and milk. The germs discharged from the mouth and nostrils are coughed up from the lungs and may infect the feed. Milk is a common source of infection for calves and hogs. Allowing hogs to run after cattle is sure to result in infection of a large percentage of them, if there are any open cases of tuberculosis in the herd. PREDISPOSING CAUSES.--Any condition that may lessen the resistance of the body or enable the tubercle bacillus to survive the exposure outside the body favors the development of the disease and the infection of the healthy animals. Crowded, poorly ventilated, filthy stables lower the disease-resisting power of the animal, and favor the entrance of the germs into the body. Under such unsanitary conditions, tuberculosis spreads quickly among dairy cattle, and a large percentage of the animals develop the generalized form of the disease. Sanitary stables and yards do not prevent the spread of the disease among animals that live in close contact with one another. Fresh air and sanitary surroundings only check the spread and retard its progress. INTRODUCTION OF TUBERCULOSIS INTO THE HERD.--The common method of introducing tuberculosis into the herd is through the purchase of animals having the disease. Such animals may be in apparent good health at the time of purchase, and be affected with generalized or open tuberculosis. _A source of infection_ is by unknowingly buying cows that have reacted to the tuberculin test. The indiscriminate use and sale of tuberculin are largely responsible for the large number of reacting animals that have been placed on the open market. This dishonest practice has resulted in the rapid spread of the disease in certain localities. For years a large percentage of the breeding herds have been infected, and the writer has met with several herds of dairy and beef cattle that became tubercular through the purchase of tubercular breeding animals.