ACTIVE AGGRESSIVITY 41 



pheric pressure; an electric current; certain chemicals, such as 

 glycerin, carbolic acid, chlorin, trichloride of iodin, potassium bichro- 

 mate, 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 specifically 

 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 

 organisms 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 estab- 

 lishment of a temporary or permanent immunity a phase of our 

 problem which will be dealt with in greater detail in a later chapter. 

 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 



