CHAPTER II 



HOST-PARASITE RELATIONSHIP 



Introduction. — The environment of man teems with potential 

 pathogenic agents. The soil frequently harbors ova of animal para- 

 sites, spores of the gas bacillus and of CI. tetani, as well as vegeta- 

 tive forms of staphylococci and other micro-organisms that survive 

 well outside of the body. It is well known that the surface of the 

 body is commonly contaminated. The skin and mucous mem- 

 branes harbor streptococci and staphylococci in great numbers, 

 while anaerobes, facultative aerobes, colon bacilli, streptococci, 

 staphylococci and many other kinds of bacteria are present in 

 enormous numbers in the contents of the intestinal tract. Rivers 

 (1928, 1932, 1933) has even found a pathogenic virus on the skin 

 of rabbits. These organisms have very little invasive power and are 

 able to enter the body only through mechanical or chemical injury 

 or as a result of altered physiological integrity of tlie mucous mem- 

 branes or skin. Theobald Smith has designated such organisms as 

 "opportunists." They are to be contrasted with the group of in- 

 fectious agents producing contagious disease and also with the 

 group of agents transmitted by insects. 



Portal of Entry. — The portal of entry into the body, as well as 

 the route of exit, is quite important in determining infection of 

 the host as well as the danger of spread of the infectious agent. 

 When the portal of entry is through the alimentary canal and the 

 exit is through the feces, as in typhoid fever, bacillary dysentery 

 and asiatic cholera, then fingers, flics, food, feces and fomites play 

 an important role in disseminating the infectious agents of these 

 respective diseases. In measles and smallpox, where the virus nor- 

 mally enters through the mucous membrane, it appears that infec- 

 tious droplets are chiefly responsible for the spread of both diseases, 

 although fluid from vesicular lesions may be a factor in the spread 

 of smallpox. In diphtheria and scarlet fever, the primary infec- 

 tion occurs in the upper respiratory tract and is spread by direct 

 or indirect contact. 



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