106 INFECTION 



strain of streptococcus will produce arthritis; another, endocarditis; 

 another, gastric ulcer, etc. 



This remarkable species and organ specificity may be due to the fact 

 that the bacteria of a particular culture have been immunized against 

 defensive forces of a particular animal host or a certain organ of the host, 

 so that, when introduced, they thrive as a result of their special and 

 acquired offensive forces. On the other hand, the specificity may be 

 due to the fact that the bacteria have been accustomed to a certain 

 nutriment furnished by a particular species or organ, and that they 

 cannot thrive unless they receive this special nutriment, and, as a result, 

 the species or organ fulfilling this requirement will become the special 

 seat of infection (Simon) . 



MIXED INFECTION 



Several different microorganisms may produce infection at the same 

 time, or one may follow the other or others and produce secondary 

 infection. The combined effects, upon the tissues of the host, of the 

 products and action of two or more varieties of pathogenic bacteria, 

 and also of the influence of these different forms on each other, are of 

 great importance in the production of disease. The metabolic products 

 of one bacteria may neutralize or accelerate the action of an associated 

 species, or combine to form a new substance entirely different from its 

 antecedents. 



Thus pyogenic cocci affect anthrax bacilli in an injurious manner; 

 on the other hand, aerobic bacteria accelerate or make possible the 

 growth of anaerobes by absorbing uncombined oxygen. Tetanus 

 bacilli will not grow outside of the body in the presence of oxygen unless 

 aerobic bacteria are associated with them; not infrequently tetanus 

 bacilli and their spores would not develop in wounds were it not for the 

 presence of the aerobic bacteria introduced with them; this factor is of 

 much importance, especially in tetanus produced by cowpox vaccine, 

 where, through careless treatment of the lesion, both tetanus bacilli and 

 pyogenic cocci' are admitted to the wound. 



Again, it may be found that one microorganism increases the viru- 

 lence of another; thus the scarlet-fever virus is favorable to the develop- 

 ment of streptococci. 



Generally all infections of mucous membranes are mixed infections. 

 Numerous bacteria are present upon the mucosa of the air-passages 

 and gastro-intestinal tract; these are usually harmless, unless the resist- 

 ance of the host is lowered in some manner, in which case not only one 



