268 INFECTION AND IMMUNITY 



depressing agencies alcoholism, insufficient nourish- 

 ment, loss of blood, etc.; or by a diminished vital re- 

 sistance at the point of invasion as a result of injury to 

 the tissues by bruising, by burns, by various chemical 

 agents, etc. 



When, however, these bacteria have been leading 

 a parasitic life for some time they have a greatly 

 increased virulence, as manifested by their ability to 

 invade the tissues of healthy individuals whenever 

 they find a portal of entrance. Under such circum- 

 stances, also, infection may result from the introduction 

 of a very small number of germs, whereas a compara- 

 tively large number would be required in case the 

 micrococci had for some time been leading a sapro- 

 phytic existence. 



Fortunately for the human race the blood serum of 

 healthy persons has the power of destroying a limited 

 number of pathogenic bacteria of a low grade of viru- 

 lence. But when these germs come directly from the 

 seat of an infectious process, especially in the case of 

 the streptococcus, a very small number may give rise 

 to a rapidly extending and deadly blood infection - 

 for example the blood-poisoning resulting from punc- 

 ture of the skin with a needle during an autopsy of a 

 case of erysipelas or of puerperal fever, or from a 

 surgical operation upon a patient suffering from any 

 form of streptococcus infection. Many pathologists 



