70 ESSENTIALS OF BACTERIOLOGY. 



CHAPTER XIII. 



INFECTION. 



How Bacteria Cause Disease. Many theories have been 

 pot forward to explain the action of bacteria in causing dis- 

 ease, but only a few of the more important ones can be taken 

 up here. 



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 conditions may cause 

 disease, but this is not an infectious disease ; it is rather a tox- 

 emia, and 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 prod- 

 ucts 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 

 in the same individual to the different forms of disease germs. 

 Susceptibility may be natural to the race, it may be acquired, it 

 may be inherited. Mice are naturally susceptible to anthrax. 

 Acquired susceptibility occurs upon exposure to conditions 



