34 BACTERIOLOGY 



mentary canal, urethra, vagina, etc., offer less resistance 

 than does the skin to the passage of germs. 



After introduction the bacteria may remain localized 

 and multiply at the point of entrance, or be rapidly 

 disseminated through the body and produce lesions at 

 distant points from the place of entrance. 



Period of Incubation. After the introduction of the 

 germ of an infectious disease into an animal organism 

 a variable time elapses before the advent of the symp- 

 toms of the disease. This period is termed the period of 

 incubation, and is fairly constant for each particular 



disease. 



SOURCES OF INFECTION 



Dust is a frequent source of infection. The bacteria 

 of tuberculosis, anthrax, influenza, and the acute infec- 

 tious diseases may attach themselves to dust particles 

 and be wafted about. 



Air. Pure air of the mountain-tops and mid-ocean 

 is free from germs. Air is, therefore, a source of infec- 

 tion just in proportion to the amount of dust, carbon, 

 and other solid particles floating in it. 



Water contaminated with sewage is a frequent source 

 of infection, especially of typhoid, cholera, and dysen- 

 tery. 



Milk is an excellent culture-medium in itself, and is 

 a source of infection in typhoid fever, scarlet fever, 

 diphtheria, and probably tuberculosis, especially in 

 infants. Hence the necessity of rigid medical inspec- 



