6 INFECTION AND RESISTANCE 



entirely iminvaded while alive, though the water surrounding it may 

 swarm with bacteria of many varieties, but when by some accident 

 such a mass of eggs ceases to live, it immediately falls prey to bac- 

 terial infection. The same point is illustrated by the rapidity with 

 which intestinal bacteria will spread throughout the body after 

 death, when during life they have remained confined to the lumen 

 of the intestine, or, at most, get into the portal circulation, to be de- 

 stroyed in the liver. By the living cell, therefore, an opposition is 

 offered to invasion by bacteria, a vital function which Bail has at- 

 tempted to make clearer by formulating it as a law, referring to it as 

 "Das Gesetz der Lebensundurchdringlichkeit." Upon what cell func- 

 tion this vital resistance to invasion depends is to a large extent a 

 mystery. It would seem to rest in principle upon the fact that the 

 invading cell meets the invaded one under conditions peculiarly 

 adapted to the activities of the latter, and is overcome before condi- 

 tions suitable for its own activities have been established. The con- 

 ditions here are not unlike those observed in the case of digestive 

 enzymes, a comparison which becomes more than an illustrative anal- 

 ogy when we consider that apart from the mere mechanical disturb- 

 ance created by the presence of bacteria as foreign bodies the struggle 

 between invader and tissue is largely one of enzyme against enzyme. 

 Thus, for instance, the gastric juice does not act upon the mucous 

 membrane of the stomach during life but after death, at autopsy, 

 partial digestion of this membrane by the pepsin is often seen. 



Whenever this vital resistance or opposition is overcome, and 

 micro-organisms enter the tissues or cells, an abnormal process is 

 taking place, and this process is, strictly defined, infection. Never- 

 theless, it is by no means necessary that such infection should al- 

 ways be accompanied by manifestations of disease. It is true that, 

 in most cases, the natural resistance is such that a struggle ensues 

 by which the invader is destroyed or thrown off, or in which the 

 invaded subject is functionally injured or even killed, and the ac- 

 companying evidences of such a struggle constitute what we know 

 as infectious disease. ' But there are special cases, cases of adapta- 

 tion, biologically speaking, in which neither invader nor host is seri- 

 ously harmed. 6 In the field of protozoology, especially, there are 

 many examples of true parasites, that is, invaders truly maintaining 

 their metabolism at the expense of the tissues and body substances 

 of the host, which do not ar,ouse reactions sufficiently vigorous to be 

 termed "disease." Thus the Trypanosoma Lewisi may be found in 

 the blood of rats 7 without noticeably affecting the health of the ani- 

 mals, and other protozoa have similarly been found in organs and 

 blood stream of a number of other apparently healthy animals. Al- 

 though such conditions have been frequently spoken of as "infection 



6 See also Bail, loc. cit. 



7 Doflein. "Die Protozoen ais Krankheitserreger." 



