38 OFFENSIVE FORCES OF THE INVADING MICROORGANISM 



acid, chlorin, trichloride of iodin, potassium bichromate, alcohol 

 etc., special care being taken, of course, to employ concentrations 

 which will not actually kill the organisms; further, by growing an 

 organism in the presence of others which tend to crowd out the one 

 under consideration; by growth in immune serum, etc. 



One additional method deserves consideration, as on first thought 

 its employment might be expected to lead to an increase in virulence 

 instead of the reverse namely, animal passage. We have pointed 

 out before that the virulence of an organism is thus usually specific- 

 ally increased for the species employed, while it remains unchanged 

 for other animals; it may happen, however, that this one-sided 

 increase is associated with an actual decrease in virulence for 

 other species. We have a practical application of this principle in 

 the attenuation of the variola virus by passage through the heifer 

 (Jenner), and in Pasteur's immunization against hog cholera by 

 passing the organism through rabbits (weaker vaccine I) and pigeons 

 (stronger vaccine II). 



Most important from a practical standpoint is the fact that organ- 

 isms which have been attenuated in their virulence through one of 

 the methods enumerated, or through still others, that have for their 

 primary object a direct impairment of the organism's resistance, will 

 either not be able to bring about an infection at all, or if this does 

 occur, a modified infection is the outcome with the establishment of 

 a temporary or permanent immunity a phase of our problem which 

 will be dealt with in greater detail in Chapter XII. The essential 

 point to be borne in mind at present is the fact that just as it is 

 possible by artificial means to increase the virulence of an organism, 

 and thus to favor the development of infection, so also is it possible 

 to bring about the reverse, and that the occurrence or non-occurrence 

 of infection must of necessity depend to a very considerable extent 

 upon the presence or absence of certain aggressive forces on the 

 part of the organism, among which the morphological evidence of 

 aggressivity is especially striking. 



Active Aggressivity. I have pointed out previously that in addi- 

 tion to such passively aggressive forces it is quite conceivable that 

 microorganisms may also possess certain active forces, and a great deal 

 of work has actually been done in the attempt to establish their exist- 

 ence. The true toxins would of course suggest themselves at once as 

 such forces, but as we have seen already, the very organisms in which 



