RELATION OF BACTERIA TO DISEASE. 95 



Bacteria also produce secondary poisons, which stand 

 a temperature of 100 to 120 C. 



The Influence of Quantity in Infection. With bacteria 

 the number introduced has an immense influence upon 

 the probability of infection taking place. 



If we introduce into a culture medium, which, like 

 the body, is only fairly suitable for growth, a few 

 bacteria, it is not improbable that they may all die; 

 whereas if a greater number are introduced, while there 

 will at first be a slight diminution of these, those that 

 die seem to neutralize the substances which were dele- 

 terious; then those bacteria which survive begin to in- 

 crease, and soon they multiply enormously. The same 

 is true for parasitic bacteria in the body. A few only 

 gaining entrance, they may die; a larger number being 

 introduced, some may or may not survive ; but if 

 a still greater quantity is injected it is almost certain 

 that there will be some surviving members, which, after 

 the destruction of antagonistic substances, and on be- 

 coming accustomed to their environment, will begin to 

 grow and produce disease. 



With those bacteria whose virulence is great i. e., 

 those which are capable of growing with great ease in 

 the body fluids a very few organisms will produce 

 disease almost as quickly as a million, allowance only 

 being made for the short time required for the few 

 to become equal in number to the million. At the 

 other extreme of virulence, however, many millions 

 may have to be introduced to permit of the develop- 

 ment of any of the organisms in the body. With these 

 bacteria we are thus able to produce either no effect 

 whatever, a local effect, or in some cases a general sep- 

 ticaemia, by regulating the amount of infection intro- 



