Infectious Agents. 63 



latter is often the case when we are deaUng with a folHcular in- 

 fection. The various pathogenic microbes do not possess uni- 

 form powers of disease production, of multipHcation and dissemi- 

 nation, granted that they gain entrance by one or other of the 

 points suggested. Some give rise to infection, no matter from 

 what point they are introduced ; that is, they are capable of 

 activity from various atria (tubercle bacilli). Others require 

 introduction in certain situations in order that they may show 

 pathogenic efitects, a fixed point of access. For example, the te- 

 tanus bacillus, as already indicated, can only be actively patho- 

 genic from wounds ; the germs of vaginal catarrh and epidemic 

 abortion naturally (partly effective also in case of intra- 

 vascular inoculations) involve only the genital mucous mem- 

 branes ; the bacteria of mastitis invade the lactiferous ducts 

 onh' ; the bacteria of calf diarrhoea confine their activity to 



- the stomach and intestines. These peculiarities probably depend 

 upon the presence at the appropriate points of entrance of es- 



* pecially favorable conditions for multiplication of the germs, 

 other parts presenting certain interfering conditions. 



Many germs require some special underlying condition in 

 order that they may live and nuiltiph'. For example, the piro- 

 plasmata must get into the blood, because they can only obtain 

 their sustenance under the conditions aft"orded by the living blood 

 and die out in the body fluids. Other organisms, as the colon 

 bacillus and vibrio of Metschnikoft', thrive best in the chyme and 

 intestinal mucous membrane. 



Doubtless the condition of the cells and juices of the body 

 plays some part in the question of development of the infection. 

 The biological and biochemical differences in cells and fluids, 

 designated as tissue predisposition and tissue immunity, are indi- 

 cated by the fact that a given species of animal is by nature com- 

 pletely resistant to a certain microbe and its toxine, surely lethal 

 for some other species, and by the varying rates with which dif- 

 ferent tissues are involved, as where practically only one of the 

 tissues offers resistance to the growth of the microbes (as the 

 resistance of muscles to tuberculosis) or where but a single tissue 

 is involved. Hence the question of development of an infectious 

 disease depends not only upon the existence of a given essentially 

 virulent microbe, but also upon the defensive powers of the body, 

 upon the antitoxic and microbicidal properties of its organs and 



