32 BACTERIOLOGY. 



organism to resist infection will be discussed more 

 fully under the subject of immunity. 



How does infection take place? It is the result of 

 the invasion of the body tissues by pathogenic bacteria 

 that live either on the surface of the body or from 

 those that live on the mucous membranes inside the 



Infec- 



tion from body. Injuries play an important part in causing 

 outside infections. Injuries caused by firearms may be the 



the body J [ J 



entering point of tetanus bacilli, the cause of lockjaw, 

 while rabies or hydrophobia is spread through the bites 

 of mad dogs. Careless manipulations with soiled 

 catheters, speculums, syringes, and so on may cause 

 injury to the tissues and be the means of introducing 

 bacteria. In the case of the contagious fevers like 

 measles, chicken-pox, whooping-cough, and scarlet 

 fever the infecting agent seems to be in the air and 

 causes infection by being inhaled. Bedding, clothing, 

 and utensils that have been contaminated with infec- 

 tious material may be the means of spreading infec- 

 tion. Finally, the bites of insects and vermin may 

 cause infection. It is known that certain kinds o<f 

 mosquitoes transmit malarial fever and yellow fever; 

 flies may spread typhoid fever by depositing the 

 typhoid bacilli on food materials. 



tio f from The bo^y ma y k e looked upon as the host for 



b fiv t ing a large numbers of bacteria. At birth, however, all 



the s body healthy animals are free from bacteria; but almost 



immediately afterward they are deposited upon the 



surface of the body by the dust in the air, and are 



introduced into the body by food and by the air 



