148 BACTERIOLOGY. 



micro-organisms bears any relation to the viru- 

 lence or activity of the substance they produce. 

 There is, however, yet another factor in the pro- 

 duction of disease. We know that in health we 

 are proof against most of these micro-organisms ; 

 if it were not so, we should all rapidly fall victims 

 to the tubercle bacillus or some others, which we 

 in health inhale with impunity. We know that a 

 microbe may only cause a local lesion in one 

 animal, and death in another. It is still more 

 striking that the same micro-organism, as is the 

 case with anthrax, may have no effect whatever 

 upon certain species of animals, though it is 

 deadly to others. Again, an animal naturally sus- 

 ceptible to the effect of a pathogenic organism 

 may be rendered proof against it. These matters 

 will be discussed in a future chapter. 



Distribution of Bacteria. Bacteria are com- 

 monly described as ubiquitous. They are ever 

 present in the air, though not in such exaggerated 

 numbers as is commonly supposed. In nutrient 

 media exposed to the air one is often astonished 

 at times at the comparatively few bacteria which 

 develop in comparison to the amount of floating 

 matter, such as mineral particles, scales, spores of 

 fungi, and debris known to be present. In water 

 they are also present in considerable numbers, 

 though of course varying according to the character 

 of the water. Wherever there is putrefaction, 

 they are present in vast numbers. In the soil, in 



