PARASITISM AND PATHOGENESIS 203 



judgment wherever these rules or similar stern logical requirements 

 have not been satisfied. 



Infectious Disease.^ — An infectious disease is a disease due 

 to the entrance of a living micro-organism and its growth in the 

 body. Although conservative bacteriologists are sometimes loth 

 to accept a disease as infectious until Koch's rules have been 

 satisfied, most are agreed that a disease, which can be reproduced 

 indefinitely by the inoculation of healthy individuals in series with 

 material taken from a preceding case, is due to a living cause. 

 The proof that a disease is due to a living cause may therefore 

 precede the identification of the causal organism, often by many 

 years. 



Possibility of Infection. — Whether a parasitic organism will 

 be able to enter and multiply in a new host and cause disease 

 depends upon a number of circumstances, the most important 

 of which may be considered under four heads, namely, the quality 

 of .the microbe, the resistance of the host, the quantity of invading 

 parasites, and the path of entrance. The course and ultimate 

 result of an infection depend also to a marked degree upon these 

 same factors. In general the qualifications of the micro-organism 

 depend first upon the experience of its ancestry under the same 

 or similar environmental conditions, factors inherent in its species, 

 and second, upon its very recent history, factors affecting the 

 virulence and general vigor of the individual microbe. Thus 

 the tubercle bacillus is qualified by inheritance for a parasitic 

 existence, while the common yeast cell is not. Yet, the tubercle 

 bacillus, when cultivated for a long time on artificial media may 

 lose its former ability to grow in the animal body. The factors 

 affecting the pathogenic properties of a microbe will be considered 

 in the succeeding chapter. 



Susceptibility and Resistance.^ — Among the important things 

 in the nature and condition qf the host, we need also to consider 

 both racial and individual characters. Certain species of animals 

 have harbored certain parasites for so long that the latter have 

 become adapted to growth in the particular species of host. In 



