234 THE BIOLOGY OF DISEASE 



a plant or animal living as a parasite in plants, animals, or 

 man. These diseases are communicated in various ways 

 from one individual to another, from one animal to an- 

 other, 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 



I 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 b}^ proto- 

 ■ ? rp -a o wi zoa (minute animals) 



Diphtheria Measles Tvphoid Scarlet Whooping ^ ' 



fever cough are malaria, yellow 

 Figure 238. — Deaths from Communicable f ever sleeoina" sick- 

 Diseases. ' ,* ° 



, , ness, possibly small- 

 This is for the year 1913 in New York r J 



State. pox, 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. 



acute anterior (infantile paralysis); puerperal septicaemia; rabies; scarlet 

 fever ; smallpox ; trachoma ; tuberculosis ; typhoid fever ; typhus fever ; 

 whooping cough. 



