96 INFECTION 



toxin produced by one species is different from that produced by another 

 in the kind of disease produced and the species of animal infected. 

 Some toxins are active for certain animals only and not for others. 

 Microorganisms of one group may possess general and common patho- 

 genic properties differing only in degree; those of different morphologic 

 and cultural characters may possess totally different powers. 



The virulence of a given species is subject to great variation. A few 

 bacteria almost constantly retain their virulence, even when kept for 

 years under artificial conditions; as an example may be mentioned the 

 diphtheria bacillus; others quickly lose their virulence as soon as they 

 are grown artificially, as, e. g., the influenza bacillus; in others and 

 probably the larger class the virulence may be raised or lowered ac- 

 cording to the experimental manipulations to which they may be sub- 

 jected. Variations may also be observed among members of the same 

 group of microorganisms, and even among individual microorganisms 

 of the same strain. 



Decrease of virulence of a microorganism may be brought about 

 artificially by repeated growth in or upon culture-media, especially 

 when transfers to fresh media are made at prolonged intervals. This 

 decrease probably depends upon an actual decrease in virulence, and 

 particularly upon the selection, in artificial growth, of the less virulent 

 or vegetative forms which grow actively and soon exceed in number 

 their more pathogenic fellows. Each time the culture is transplanted 

 more of the vegetative and fewer of the pathogenic microorganisms 

 are carried over, until finally the pathogenic bacteria are entirely elim- 

 inated, or their virulence totally destroyed, and the entire culture is 

 composed only of vegetative or harmless forms of bacteria. 



Various other agencies lead to artificial lessening of virulence, such 

 as exposure, for short periods of time, to a temperature just under the 

 thermal death-point; exposure to sunlight; exposure to small quantities 

 of antiseptic or germicidal substances; the action of desiccation; sub- 

 jection to increased atmospheric pressure, etc., these methods being com- 

 monly employed in the preparations of vaccines to be used for purposes 

 of active immunization. 



The passage of a microorganism or virus through animals usually 

 increases its virulence, but may modify or attenuate it, as in the case of 

 the passage of smallpox virus through the calf, when it loses forever its 

 power of producing smallpox. 



Increase in virulence can best be secured by passing the microorgan- 

 ism through animals. It is practically impossible, by any means, to 



