It will be readily understood that the great majority of bacteria present no relation whatever to disease; they have their various functions in the economy of nature, and may never come into relation with the living body of an. animal. Their function in nature seems to be chiefly to prey upon dead animal and vegetable matters, and so to cause their disintegration. In this view, the term Saprophytes ( = putrid) is used. Some of the bacteria on the other hand are capable of living in the bodies of higher organisms, either animal or vegetable, and of preying on their tissues or juices. In this sense they are properly called Parasites. Even when living in the body of an animal or plant a microbe may leed merely on the dead injesta or excreta, and may therefore be purely saprophytic. In the living body there are continually present multitudes of microbes which are its normal tenants and exercise their saprophytic functions. In the alimentary canal they are even supposed to aid digestion and absorption by splitting up proteids and carbohydrates which have escaped the action of the digestive juices.
On the other hand certain microbes are abnormally present in the tissues and canals of the body, and are capable, by means of their toxic products, of giving rise to various forms of disease, such as fevers, inflammations, etc. Microbes of this kind are designated Pathogenic, as contrasted with the Non-pathogenic which produce no such results. It must be added that a microbe which is saprophytic in the skin or alimentary canal may be highly pathogenic when introduced into the tissues. It is proper to designate a microbe as pathogenic if it is capable of producing disease under suitable circumstances, although for the most part it may be purely saprophytic.
Bacteria produce their pathogenic effects by means of the toxic substances which they evolve (see p. 337). As they are in themselves small particles of matter, often very insignificant in relation to the mass of the tissue, their mere presence can produce little disturbance. Nor is it by using up the material required by the tissues that they do harm. In a few cases, such as that of the anthrax bacillus, in which they are present in enormous numbers in the blood, it has been supposed that they do harm by using up the oxygen of the blood, or blocking the capillaries of the lungs so that respiration is impeded. But if at all true this view is only partial. Bacteria produce their effects essentially by the products which they evolve, and the effects vary according to the kind of bacteria and the nature of the products. This subject has already been referred to.