CHAPTER XI 

 THE PATHOGENIC PROPERTY OF MICRO-ORGANISMS 



Adaptation of Parasitism. — In order to live as a parasite, an 

 organism must be adapted, to grow under the conditions met 

 with in the body of the host^ but in order to produce disease it 

 must also injure the host. The most perfect adaptation of 

 parasitism is probably exhibited by those micro-organisms 

 which do not injure the host, the symbionts and commensals, 

 as it is obviously to the interest of the parasite to keep its host 

 alive. An adaptation of this kind usually requires that the 

 microbe shall either grow very slowly, or shall be so situated 

 that the excessive numbers resulting from its multiplication 

 may readily pass out of the host or be disposed of in some way; 

 otherwise the host would be physically crowded out. This sort 

 of adaptation is illustrated by the normal intestinal bacteria. 

 Parasites which invade the tissues of the body rarely show such 

 adaptation.. It is, perhaps, approached to some extent by 

 the slow-growing bacilli of leprosy and tuberculosis. In most 

 instances of parasitism, however, there is more or less of a struggle 

 between the invader and the host for the possession of the field, 

 and the phenomena Of disease are incident to this combat. 



Virulence.^ — The ability of the parasite to injure its host, is 

 designated as virulence. Virulence depends in part upon growth 

 vigor, but also upon other factors largely unknown. A great 

 deal is known about specific methods of changing the virulence 

 of micro-organisms, and various procedures are commonly em- 

 ployed with this object in view. A diminution in virulence is 

 called attenuation and an increase in virulence, exaltation. 

 Attenuation was first observed by Pasteur in a culture of Bacterium 

 avisepticum (chicken cholera) grown in broth in the presence 



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