INFECTION 75 



CHAPTER XIII 

 INFECTION 



How Bacteria Cause Disease. Many theories have been 

 advanced to explain the action of bacteria in causing disease, 

 but only a few of the more important ones can be taken up here. 

 Nearly all the changes found in the organs of the body are 

 similar to those produced by drugs and can be reproduced 

 by the injection of bacterial poisons. 



What are the Conditions Necessary to Produce Infection? 



First, as to the infective agent. The organism must have 

 the power to produce disease. It must, in other words, be 

 pathogenic. A non-pathogenic bacterium under certain con- 

 ditions may cause disease, but this is not an infectious disease; 

 it is due to the absorption of poisons generated outside of the 

 body. It must be parasitic have the power of growing within 

 the body of an animal. 



Essentially an infectious disease is a toxemia, because it 

 depends upon poisons or toxins produced in the body. Para- 

 sitic or infectious bacteria cause disease by growing in the animal 

 organism and generating products therein which are toxic. 

 Saprophytic bacteria grow outside of the animal organism in 

 dead matter, decaying particles, etc., and they may give rise 

 to products which also are toxic to the animal economy. 



Second, the toxins or poisons elaborated must be present in 

 sufficient amount. Undoubtedly each animal organism is a law 

 unto itself in regard to the amount of poison it will tolerate 

 before disease is actually produced. The period of incubation 

 can be explained on the supposition that the germ requires so 

 much time to elaborate the amount of toxin necessary. This 

 time period varies with different organisms, some carrying the 

 toxin with them at the time of entry. 



Third, the animal infected must be susceptible. Susceptibility 

 varies in different species of animals, in different members of 

 the same species, in the same individual at different times, and 



