CHAPTER LXXI 

 BACTERIA AND HEALTH 



428. Bacteria and specific diseases. Before germs can cause 

 disease it is necessary that they enter the body of the host. 

 Ordinarily they cannot get through the skin. The infection, or 

 entrance into the body, therefore, takes place through either 

 (i) a cut in the skin or (2) one of the regular openings to the 

 interior of the body, as the mouth or the nose. 1 



Fortunately for us, most bacteria do not cause disease. We 

 may therefore carry about with us, in our mouths and air pas- 

 sages and food tubes, millions of bacteria without being made ill. 



After the middle of the last century the improvements in the 

 microscope and the development of experimental methods made 

 possible the discovery that certain diseases are caused by mi- 

 crobes, and that they can be caused in no other way. Since the 

 time of Pasteur, the French chemist who first demonstrated this 

 idea, many physicians and biologists have succeeded in finding 

 the particular species of bacteria connected with some of the 

 most important human diseases, such as tuberculosis, diphtheria, 

 pneumonia, typhoid fever, tetanus (lockjaw), cerebrospinal men- 

 ingitis, and others. The methods developed in the course of 

 these studies have been successfully used in the treatment and 

 prevention of several other diseases, although the specific or- 

 ganisms that cause these are not known in all cases. For, in 

 addition to discovering that a given species of bacteria is the 

 specific cause of a disease (for example, the typhoid bacillus in 

 the case of typhoid fever), we have found (i) that the bacteria 



1 In some cases the bacteria may act upon tissues without penetrating into 

 the interior of the host, as the diphtheria germ, which lives on the mucous 

 surface of the throat. 



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