456 VETEEINA.EY HYGIENE 



The effect of pathogenic bacteria on the system is due 

 either to their development in the tissues or to the 

 absorption of their toxins. The soluble toxins readily 

 enter the system, the insoluble ones escape only with 

 the disintegration of the cell. 



Certain bacteria act chiefly by their toxins ; others are 

 not able to live in the tissues though their toxins are very 

 fatal. Some bacteria can live in wounds but do not 

 invade the tissues, though their toxin does, and this latter 

 produces death by poisoning, as for example, the bacillus 

 of tetanus ; other bacteria can only cause a circumscribed 

 pathological condition, and this is brought about by the 

 toxin being absorbed by the adjoining cells. 



The toxin of all pathogenic microbes is not known, but 

 they are all assumed to exist. Death by microbial infection 

 may be due to the action of toxin, occasionally to the 

 bacteria withdrawing nutriment from the tissues, and fre- 

 quently to the mechanical effect of the organisms in blocking 

 the capillary vessels ; probably in most cases two or more 

 of these influences are at work. 



The pathogenic effect of a bacterium depends upon 

 varying conditions, the principal one being its virulence, 

 while the latter depends upon its toxic effect and reproduc- 

 tion. The virulence of an organism is a varying factor 

 easily influenced by several causes, for instance, the amount 

 of the virus and the method of its application. 



Some bacteria are increased in virulence by being mixed 

 with harmless bacteria ; others become pathogenic by 

 chemical or mechanical alterations in the tissues, and 

 some will only grow when a suitable condition of body has 

 been produced by a prior infection. 



The development of two or more organisms in the body 

 is termed a mixed infection ; successive infections by t he 

 same organisms are called secondary infections. 



Infection may result through wounds, inhalation, food, 

 water, bites of insects, and coitus. 



Infection may be direct, and this is the most common 

 condition; in some diseases it is indirect, the organism 



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