l6 INCREASED VIRULENCE 



whilst those which are isolated from the lungs in pneumonia, or 

 from pneumococcic lesions in general, are usually far more virulent. 

 Other explanations are possible, but it seems likely that the 

 sequence of events is as follows : The avirulent pneumococci gain 

 access to the body owing to a temporary loss of immunity, due to 

 one or other of the causes enumerated above, and then these are 

 transmitted to a process in all respects like passage, the result 

 being that they undergo a gradual increase in virulence. The 

 struggle of the conservative forces will then be increasingly difficult, 

 and the patient may succumb to an infection with an organism 

 which was at first but slightly virulent. This adaptation of an 

 organism to its environment during the course of a disease may 

 probably be found in the future to be of great importance, as indi- 

 cating a necessity for successive changes in the vaccine or serum 

 used in the treatment of a chronic infection. 



An example worthy of notice has recently been given by 

 Ehrlich. It is not exactly on the same lines, since it deals with 

 an alteration in the body of the power possessed by the parasite of 

 resisting chemical agents of relatively simple composition, rather 

 than in the power of resisting the natural forces of the body, an 

 increase in which constitutes an increase in virulence. Ehrlich 

 investigated the preventive and curative action of atoxyl and of 

 various aniline dye-stuffs, such as fuchsin and trypanroth, on mice 

 infected with trypanosomiasis. He found in a certain number of 

 cases a cure might be obtained e.g., by feeding infected mice with 

 fuchsin or by the injection of atoxyl and that when this occurred 

 the trypanosomes were not entirely destroyed, but remained latent 

 in the body. This is a phenomenon of fairly frequent occurrence, 

 and is called by Ehrlich, " immunitas non sterilisans." After a 

 time a relapse occurred, and was cured by a fresh dose of the drug, 

 but after several of these recurrences this beneficial effect ceased. 

 It was then found that the trypanosomes had been immunized or 

 acclimatized to the agent in question say, to fuchsin and pos- 

 sessed the power of infecting mice previously treated with fuchsin 

 and immune to ordinary trypanosomes ; but the organism had 

 not altered in its susceptibility to other dye-stuffs or to atoxyl, 

 and mice infected with it could be cured by these agents, and not 

 by fuchsin. Further, it was found possible to create a race of 

 trypanosomes resistant to two or more of these agents, and these 

 acquired characters were made permanent after several passages. 

 If we substitute for the drugs used by Ehrlich the substances 



