CHAPTER VI. 



INFECTION PHAGOCYTOSIS OPSONINS. 

 INFECTION. 



WHEN pathogenic bacteria enter an animal by one of the routes 

 indicated, and multiply in the cavities, organs, or tissues, and by so 

 doing produce disease, an infection has taken place, and the disease 

 produced is called an infectious disease. When a disease of this type 

 can be transferred directly from one animal to another it is called 

 contagious. Tuberculosis and glanders, for instance, are contagious 

 diseases; but actinomycosis, while infectious, is not contagious, 

 because there is no proof that it is ever directly transferred from one 

 animal to another; the infective and infecting agent, the actinomyces 

 fungus, being always taken up with the fodder. Texas fever is an 

 infectious disease, but it is not contagious. However, even purely 

 infectious non-contagious diseases may be directly transferred from 

 the parent to the offspring through the placental circulation. 



Toxins. The question arises, How do microorganisms produce 

 disease? Is it by their mere presence? Occasionally bacteria may 

 so multiply that they accumulate in the capillaries, where they may 

 lead to bacterial thrombi or emboli; but this is very exceptional. 

 As a rule, bacteria produce disease by forming substances highly 

 poisonous to the host in which these parasites multiply. These sub- 

 stances are called toxins. When a venomous snake bites it is not 

 the small wound which causes disease and death, but the venom which 

 gets into the circulation. In the same way it is not the presence of 

 bacteria in the tissues and juices, but the toxins which they manu- 

 facture in their physiologic processes that, as a rule, cause disease. 



Types of Toxins. We must distinguish between two types of 

 toxins, the extracellular or soluble toxins, and the intracellular or 

 insoluble endotoxins. 



Some pathogenic bacteria, for instance the tetanus bacillus and the 

 diphtheria bacillus, in their growth in artificial media form soluble 

 extracellular toxins. These get into the fluids in which the bacilli 

 grow and the latter can be removed by filtration, so that a fluid is 

 obtained containing the soluble toxins only. On the other hand, if 

 colon, typhoid, or anthrax bacilli, or cholera spirilla, are grown in 

 fluid culture media no toxin will be found in the fluid. Finely 

 divided or decomposed bacteria of this kind must be disinte- 

 grated before it is possible to obtain their intracellular, ordinarily 



