PULMONARY TUBERCULOSIS 
489 
man. These diseases are communicated in various ways 
from one individual to another, from one animal to another, 
or from one plant to another. 
The following are among the most common communi¬ 
cable diseases. Diseases caused by bacteria (minute plants) 
are tuberculosis, pneumonia, diphtheria, typhoid fever, 
bubonic plague, and whooping cough. Measles and scarlet 
fever are so similar to these in many ways that it is believed 
that they are caused by bacteria, although the definite 
bacteria which cause them have not been discovered. Dis¬ 
eases caused by protozoa (minute animals) are malaria, 
yellow fever, sleeping sickness, possibly smallpox, and others 
less well known. 
The biological diseases are all preventable, especially 
the communicable diseases which result from the parasitic 
habit of some plant or animal. In order to prevent these 
diseases, it is necessary to know how the different plants 
and animals gain access to the human body and proceed 
to live there. This can be illustrated by describing pul¬ 
monary tuberculosis, a plant or bacterial disease; and 
malaria, an animal or protozoan disease. 
375. Pulmonary Tuberculosis. — Pulmonary tuberculosis 
is a disease located in the lungs. The cause is a definite 
plant with parts and habits which are easily recognized by 
bacteriologists (students of bacteria). This plant is called 
Bacillus tuberculosis , and was proved to be the cause of 
consumption, or tuberculosis, by Robert Koch, a German 
scientist, in 1882. These tuberculosis bacteria, or germs, 
in countless numbers are found leading a parasitic life in 
the lungs of a tubercular patient. The bacteria are ex¬ 
tremely minute, and can be seen only by the use of a micro¬ 
scope of high power. 
The large number of germs in the lungs grow rapidly 
and they are set free in the air by coughing. One tuber¬ 
culosis patient may give off millions of these germs in a 
