266 INFECTION AND IMMUNITY 



abscesses, in boils and carbuncles, in suppurating 

 wounds, in puerperal fever, in peritonitis, in suppur- 

 ative disease of the ear, in general blood-poisoning, 

 etc. 



As in other infectious diseases infection by these 

 pathogenic micrococci (so-called " pus cocci ") depends 

 upon three factors, viz. : the virulence of the germ ; 



FlG. 10. Micrococcus of pus-formation {Staphylococcus pyogenes aureus) ; 

 magnified 1000 diameters. 



the vital resistance of the tissues invaded ; and the 

 number of germs introduced into an open wound. 

 The virulence of the germs is much greater when 

 they come from a suppurating wound, from a case of 

 erysipelas or of puerperal fever, or, in short, from any 

 infectious process in the body, than when they come 

 from the mouth or the surface of the body where they 



