124 INFECTION AND IMMUNITY. 



may be a more favorable medium for their exist- 

 ence. Thus, they increase in virulence, and disarm 

 the host by becoming resistant to its protective 

 agencies, and even actively antagonize the latter, 

 particularly as regards phagocytosis, and by such 

 means the natural immunity of the host is rendered 

 inefficient. The host, however, is not without its 

 own reserve forces, and, in favorable cases, after 

 infection is once established, it responds to the pres- 

 ence of the micro-organisms by the production of 

 a new supply of protective "antibodies," the effect 

 of which is to destroy larger numbers of the invad- 

 ing organisms, or even to sterilize the body com- 

 pletely. In the latter case recovery is the prob- 

 able event. This, again, may be looked on as a 

 process of adaptation on the part of the host, in 

 which it seeks to destroy or render less noxious the 

 infecting microbes. 



Mutual In certain chronic infections it seems that a 

 mutual adaptation takes place, so that there is a 

 tendency for the host and parasite to live in a half- 

 way state of harmony. Thus, in the acute stage 

 of s}^philis, the "combat" between the host and the 

 spirochetes is an active one. Eventually, however, 

 it would seem that the host reacts by the produc- 

 tion of protective antibodies, a condition which 

 results in an amelioration of the symptoms. Since 

 the micro-organisms may remain alive and viru- 

 lent in the host for a long time after this has 

 occurred, it appears that they become habituated 

 to the presence of the antibodies, while the host 

 at the same time becomes habituated to the toxicity 

 of the spirochetes. After the lapse of still further 

 time the adaptation on the part of the patient 

 appears to increase, and the micro-organism also 



