66 PATHOGENIC BACTERIA. 



or not these organs can permit their exit without being 

 themselves diseased is a question not yet solved. 



If the infecting organism belongs to the important 

 class of pathogenic bacteria, it is still in many cases 

 uncertain just what results will follow its entrance into 

 the tissues, for so many modifying elements may be pres- 

 ent that the effects observed on one occasion may be very 

 unlike those observed on another, notwithstanding the 

 fact that the conditions under which the observation was 

 made seem to be the same. 



The streptococcus infections may be used as an ex- 

 ample of this variation. In one case infection by this 

 micro-organism may produce a circumscribed abscess of 

 mild variety, in which we are surprised to find so impor- 

 tant a germ; at another time, the infection leads to an 

 attack of erysipelas; at still another, we find it the cause 

 of septic infection and death. Parallel results are often 

 obtained from the same culture when injected into ani- 

 mals. The recently isolated streptococcus injected into 

 'the ear vein of a rabbit causes death by septicemia; after 

 a few transplantations it may no longer cause septicemia, 

 but an erysipelatous inflammation of the ear; rarely sup- 

 purative lesions follow its subcutaneous injection into 

 rabbits. 



CONDITIONS MODIFYING THE RESULTS OF INFECTION. 



I. The Infecting Bacterium. 



i. Virnle7ice is the disease-producing capacity of a micro- 

 organism. It is a most variable condition, and probably 

 no two cultures of the same micro-organism are identical 

 in their pathogenic power. Virulence depends upon 

 biological peculiarities, such as adaptability to environ- 

 ment, which permit the bacteria to grow rapidly in the 

 body, and the preponderance of the toxin-forming 

 function over the purely vegetative function, etc. So 

 variable is the virulence of micro-organisms of the same 

 species that it is almost impossible to establish a standard 

 l>y which to compare them. As soon as the micro- 



