30 BACTERIOLOGY. 



organism is called infectious. As examples of infec- 

 tious disease, diphtheria, pneumonia, influenza, tuber- 

 culosis, and syphilis may be mentioned ; although there 

 are many others. A contagious disease is one that is 

 transmitted from one person to another through the 

 air or by simply coming into 1 the presence of or touch- 

 ing the sick. Smallpox, scarlet fever, measles, chicken- 

 pox, and German measles are contagious. All con- 

 tagious diseases are infectious, but not all infectious 

 diseases are contagious. Diseases like cholera, gland- 

 ers, pneumonia, plague, tuberculosis, and syphilis 

 cannot be transmitted through the air or by coming 

 into the presence of the sick. Typhoid fever may be 

 considered infectious through water and other infected 

 foods, and contagious by contact with the so-called 

 typhoid carriers. 



The terms "infestation" or "infestion" are ap- 

 plied to diseases caused by the entrance into the body 

 of large parasites such as amebae, worms, and so on. 

 Factors While the presence of pathogenic bacteria is 



Ti?g e in~ necessary to cause infection, other factors of much 

 importance must be taken into consideration. This 

 must be so, as every-day experience shows. In any 

 epidemic of infectious disease only a portion of those 

 exposed become infected. Even among those infected 

 the disease presents all variations from the very mild 

 to the most severe. The factors that influence the 

 onset and course of infections relate both to the bac- 

 teria and the individuals exposed to them. 



So far as the bacteria themselves are concerned, 



