84 INFECTION 



terms are merely relative, and bacteria ordinarily saprophytic may 

 develop parasitic and pathogenic powers when the resistance of the host 

 is sufficiently reduced by another infection, fatigue, exposure, or other 

 deleterious influence. In other words, a pathogenic microorganism 

 is one that can grow in the living tissues because the immunologic 

 defenses of the host are not sufficiently strong to resist it; in most cases, 

 however, as will be pointed out further on, a higher degree of immunity 

 can be produced artificially, rendering the bacterium in question rela- 

 tively harmless for that particular animal. Similarly, under certain 

 circumstances, the resistance of the body or of a part of it may be broken 

 down to such an extent that microorganisms ordinarily regarded as 

 saprophytes may gain access to the deeper tissues, flourish, and produce 

 disease. 



Accordingly, no fundamental distinction between pathogenic and 

 non-pathogenic bacteria can be made. Any apparent differences are 

 due not only to various degrees of pathogenicity possessed by the micro- 

 organism, but also to the different degrees of resistance against their 

 attacks, since a microparasite that is highly pathogenic toward one 

 animal, may be quite harmless to another. 



CONTAGIOUS AND INFECTIOUS DISEASES 



Just as all pathogenic bacteria do not possess the same habits of 

 growth, so, likewise, they vary in their vitality and in their ability to 

 proliferate under various conditions when removed from the animal 

 body. Some are strictly parasitic, and are able to grow only at body 

 temperature, or, indeed, only in the human body itself; when removed 

 from these conditions, they may retain their vitality for a short period 

 of time, but are unable to proliferate: from this it follows that com- 

 munication of these bacteria and their disease must be direct or immedi- 

 ate, i. e., from person to person, or almost direct, by the conveyance of 

 the infecting agent in the form offomites, such as dust, epidermal scales, 

 or discharges, or as the result of bites of suctorial insects. This form 

 of infection, which requires such direct means of transmission, and of 

 which gonorrhea is an example, constitutes what are known as conta- 

 gious diseases. 



Other microparasites are not so strictly parasitic; they may be able 

 to preserve their pathogenic powers and proliferate outside of the body 

 at ordinary temperatures, and may even withstand great extremes of 

 heat or cold and various nutritional deficiencies; they may exist thus 

 for weeks, and carry the disease to a second individual through con- 



