168 INFECTION AND IMMUNITY. 



The modern conception of infection implies the presence in the body 

 of the living micro-organisms themselves, that is, of something capable of 

 multiplication, or at least of reproduction and development, and not alone 

 of the poisons which they may and usually do produce. It is customary 

 to look upon the effects of the absorbed poison which micro-organisms 

 furnish as intoxications, whether these poisons be formed inside the body 

 in infectious diseases and in other conditions, or outside of it and subse- 

 quently introduced. That condition in which there is evidence of wide 

 distribution of pathogenic micro-organisms and their products in the 

 blood is called septiccemia (see p. 202). 



It is evident from what has been said that infectious disease cannot 

 exist without the presence in the body of micro-organisms. But, on the 

 other hand, micro -organisms can and do frequently exist in the body 

 without the incitement of infectious disease. Whether a micro-organ- 

 ism be pathogenic or not depends upon the variable susceptibilities of 

 the host as well as upon its own also variable nature and qualities. 

 No micro-organism is intrinsically pathogenic ; the very conception im- 

 plies a relationship. This obvious fact is overlooked by those who see in 

 the micro-organisms alone the essential specific features of infectious 

 diseases ; who would classify these diseases exclusively by the nature of 

 their excitants, and who look upon the latter as the true "causes" of the 

 phenomena through which disease is manifested (see p. 67). 



FORMS OF INFECTION. 



Mixed or Concurrent Infection. It should be borne in mind that the 

 body which is already the seat of an infectious disease is usually espe- 

 cially susceptible to the action of other pathogenic germs, should these 

 once gain entrance ; and also that the lesions which are associated with 

 many of the infectious maladies afford portals of entry through the skin 

 or mucous membranes to other micro-organisms, against the entrance of 

 which the healthy body opposes most efficient barriers. In fact, we now 

 know that the action of two or more pathogenic micro-organisms in the 

 body at the same time is of very frequent occurrence, many of the so- 

 called complications of the infectious diseases being due to secondary 

 infection with a new germ species. Numerous examples of this "mixed " 

 or, better, "concurrent," infection are noticed in other parts of this 

 book. ' 



Many important facts have been revealed by the study of bacterial 

 association in cultures as well as in infectious diseases of men and ani- 

 mals which cannot here be considered. 2 It may be said in general that 

 in animals as in man concurrent infection with a second micro-organism 

 increases the gravity of the original situation. On the other hand, cer- 

 tain experiments seem to indicate that sometimes the concurrent action 



'For bibliography of mixed infection see Bernheim and timber, Lubarsch and 

 Ostertag's "Ergebnisse," Jahrg. 2, for 1895, p. I; also Wasxermann, in Kolle and Was- 

 sermann's "Handbuch der Mikroorganismen," Bd. i., p. 307. 



* Consult Tli. Smith, Trans. Association American Physicians, vol. ix., p. 85, 1894. 



