90 INFECTION AND IMMUNITY. 



disease in a wide range of animals. Such microbes 

 are the pyogenic cocci, the bacilli of tuberculosis, 

 glanders, typhoid fever, cholera, paratyphoid fever, 

 tetanus, diphtheria, plague, certain of the trypano- 

 somes, and many others. There are examples of 

 immunity, however, even to these micro-organisms 

 of rather general virulence, such as that of the alli- 

 gator and the fowl to tetanus, and of the rat to 

 diphtheria. 



In contrast to the examples mentioned above, 

 are the non-pathogenic parasites, which experi- 

 mentation has shown to have no, or insignificant 

 virulence for any animal whatsoever. B. subtilis 

 and B. megatherium are cited as representing such 

 organisms. The former, however, is not absolutely 

 without pathogenic power, since it has been observed 

 as the cause of panophthalmitis in a number of 

 cases, and by previously lowering intraperitoneal 

 resistance (by the injection of "aggressins") Weil 

 caused fatal peritonitis in guinea-pigs with this 

 organism. 



Standing somewhat above these more or less 

 harmless microbes in pathogenic power are the so- 

 called acid-fast grass bacilli and similar organisms, 

 which are able to produce only slight local changes 

 in animals, and which are soon destroyed by the 

 antibacterial agencies of the host. 



variations. Virulence is a variable factor even with regard 

 to different strains of a given micro-organism. 

 Diphtheria bacilli cultivated from various cases 

 show a rather wide range of virulence, and similar 

 observations have been made with regard to typhoid 

 bacilli, streptococci, pneumococci, tubercle bacilli, 

 plague bacilli and other organisms. With few ex- 

 ceptions, pathogenic micro-organisms tend to lose 



