GENERAL SUSCEPTIBILITY IN RELATION TO INFECTION 99 



are led to the very important conclusion, laid down by Adami, that "in 

 infections the body is never involved as a whole. Coineidentally with 

 the growth of the specific germs in individual organs there tends to be a 

 reaction to, and destruction of, the same in other parts." 



The numeric relationship of bacteria to infection is very important, 

 and the number alone may determine whether or not it shall occur. 

 Usually the normal defensive factors of the body are sufficient to over- 

 whelm one or a few bacteria unless they are especially virulent. When 

 an intercurrent or chronic disease, malnutrition, or injury renders the 

 host more susceptible than normal, fewer bacteria than would other- 

 wise be required may successfully infect the body. Also with true 

 parasites, or those with well-marked aggressiveness, such as the anthrax 

 bacillus, a few may be sufficient, if they reach the circulating fluids, to 

 produce infection. Thus Webb, Williams, and Barbor 1 have found that 

 one anthrax bacillus was sufficient to infect a white mouse, and as few 

 as 20 tubercle bacilli were sufficient in one instance to infect a guinea-pig. 



Park has directed attention to the fact that when bacteria are trans- 

 planted from culture to culture, under supposedly favorable conditions, 

 many of them die; it is highly probable that when they are transplanted 

 to an environment that is likely to be unfavorable, as are the body 

 tissues with various defensive mechanisms, many more must die. This 

 is an important point to bear in mind in attempting to correlate experi- 

 mental results with the natural cause of an infectious disease. In the 

 laboratory we reproduce disease experimentally by the immediate in- 

 jection of millions of bacteria, whereas in nature there is rarely any such 

 immediate overwhelming of the tissues. For example, pneumonia may 

 be produced experimentally in dogs by the injection of a large number of 

 virulent pneumococci directly and at once into the bronchi, yielding a 

 positive result with a microorganism which, under natural conditions 

 and in smaller numbers, would be relatively innocuous for the animal 

 under observation. 



GENERAL SUSCEPTIBILITY IN RELATION TO INFECTION 



Under normal conditions the body-cells of a host will invariably 

 offer some resistance to invasion and infection by pathogenic micro- 

 organisms. When, however, any condition that depresses or diminishes 

 general physiologic activity and vitality exists, the host may be unable 

 to master these defensive forces, and accordingly becomes predisposed 

 or more susceptible to infection. 



1 Transactions Sixth International Congress on Tuberculosis, 1908, p. 194. 



