196 GENERAL BIOLOGY OF MICRO-ORGANISMS 



the wisdom of reserving final 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 enviromental 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 of 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 



