The disease called tuberculosis is caused by a germ known as the tubercle bacillus, whenever the body's condition is such that the germ can multiply and do its destructive work in the bodily tissues and organs.The germs live for hours in moist sputum outside the body, and may survive drying and freezing for long periods. They die after a few minutes in boiling water, and do not live long in sunlight or fresh air. But once they get a foothold in the body they can multiply very rapidly.Tuberculosis can attack any part of the body, but pulmonary (lung) tuberculosis is by far the commonest form of the disease in human beings in this country, and this chapter will discuss only pulmonary tuberculosis.Infection takes place in the great majority of cases by breathing in tubercle bacilli, whether in dust or in moist droplets. Lodged in the furthermost recesses of the lung, they can remain unmolested to produce the typical local lesions of tuberculosis when conditions are favorable.Bacilli that are swallowed may also cause infection. These are subject to the chemical action of the digestive juices, and to mechanical action by which they are washed out of the body in the intestinal discharges; it requires larger numbers of bacilli to produce infection by swallowing than by inhalation.The germ can be passed from one person to another in a number of ways. By far the most frequent mode of transmission is continued close contact with a person who has active tuberculosis. Such a person has living bacilli in the sputum and in the tiny droplets sprayed from the mouth and nose in coughing, sneezing, and even in laughing or talking forcibly. If anyone comes in contact with this spray, he may inhale the bacillus-loaded droplets. Kissing a person with active tuberculosis also is dangerous because the bacilli may be planted directly on the lips. The above are examples of what is called direct contact.