In the first place, we notice that a majority of bacteria are utterly unable to grow in the human body even if they do find entrance.  There are known to bacteriologists to-day many hundreds, even thousands of species, but the vast majority of these find in the human tissues conditions so hostile to their life that they are utterly unable to grow therein.  Human flesh or human blood will furnish excellent food for them if the individual be dead, but living human flesh and blood in some way exerts a repressing influence upon them which is fatal to the growth of a vast majority of species.  Some few species, however, are not thus destroyed by the hostile agencies of the tissues of the animal, but are capable of growing and multiplying in the living body.  These alone are what constitute the pathogenic bacteria, since, of course, these are the only bacteria which can produce disease by growing in the tissues of an animal.  The fact that the vast majority of bacteria can not grow in the living organism shows clearly enough that there are some conditions existing in the living tissue hostile to bacterial life.  There can be little doubt, moreover, that it is these same hostile conditions, which enable the body to resist the attack of the pathogenic species in cases where resistance is successfully made.

What are the forces arrayed against these invaders?  The essential nature of the battle appears to be a production of poisons and counter poisons.  It appears to be an undoubted fact that the first step in repelling these bacteria is to flood them with certain poisons which check their growth.  In the blood and lymph of man and other animals there are present certain products which have a direct deleterious influence upon the growth of micro-organisms.  The existence of these poisons is undoubted, many an experiment having directly attested to their presence in the blood of animals.  Of their nature we know very little, but of their repressing influence upon bacterial growth we are sure.  They have been named alexines, and they are produced in the living tissue, although as to the method of their production we are in ignorance.  By the aid of these poisons the body is able to prevent the growth of the vast majority of bacteria which get into its tissues.  Ordinary micro-organisms are killed at once, for these alexines act as antiseptics, and common bacteria can no more grow in the living body than they could in a solution containing other poisons Thus the body has a perfect protection against the majority of bacteria.  The great host of species which are found in water, milk, air, in our mouths or clinging to our skin, and which are almost omnipresent in Nature, are capable of growing well enough in ordinary lifeless organic foods, but just as soon as they succeed in finding entrance into living human tissue their growth is checked at once by these antiseptic agents which are poured upon them.  Such bacteria are therefore not pathogenic germs, and not sources of trouble to human health.