CHAPTER XIII. 



THE RELATION OF BACTERIA TO DISEASE. 



Ix preceding chapters we have considered the chemical effects of 

 bacteria and their ferments on dead organic substances. Now we have 

 to consider the growth of bacteria in the living host and the results of 

 such development. While it is true that there is a great difference 

 between living and dead matter, and that, therefore, the living animal 

 cannot be looked upon as merely a quantity of peculiarly specialized 

 material to be used for food for bacterial growth, still, in a very 

 real sense, we are warranted in considering the infected living body 

 as a food mass subject to bacterial growth. The difference is that 

 besides the chemical substances, temperature, and conditions inherent 

 to the fluids of the living body and its tissues, micro-organisms have 

 also to reckon with the constant production of new substances by the 

 living cells of the invaded organism, which maybe antagonistic to them. 

 In the production of lesions by micro-organisms there are four main 

 factors involved viz., on the part of micro-organisms, the power to 

 elaborate poison and the ability to multiply; on the part of the body, 

 the degree of sensitiveness to the poisons of the bacteria and the tendency 

 to produce antitoxic or bactericidal substances. Xo known variety of 

 bacterial cell has as a single organism the ability to produce enough 

 poison to do appreciable injury in the body, nor is it probable that there 

 is any variety which, if it multiplied in the body to the extent that some 

 pathogenic bacteria are capable of, would not produce disease. As 

 already mentioned, varieties of bacteria even under similar conditions 

 differ enormously in the amount of poison which they produce and in 

 their ability after gaining entrance to multiply in the body. 



To understand the bacterial factor in the production o*f disease we 

 must recognize that both the body invaded and the bacteria which 

 invade are living organisms, arid that the products of the cellular activity 

 of the body act on the bacteria at the same time the bacterial products 

 act upon the human body. Just as there are different races and species 

 of animals having dissimilar characteristics, there are different races 

 and species among bacteria, and just as the descendants of one animal 

 species under changing conditions gradually become diverse, so do the 

 descendants of one bacterial species. In fact, the rapidity of the devel- 

 opment of new generations of bacteria allow in them of much quicker 

 changes under new conditions than are possible in the higher animals 

 and plants. Considering these and other facts, we can readily under- 

 stand how the different types of bacteria do not grow equally well in 

 every variety of animal, and after discovering that there are variations 



