The difficulty of giving such a list is rendered greater from the fact that we have in recent years learned that the same species of pathogenic bacterium may produce different results under different conditions.  When the subject of germ disease was first studied and the connection between bacteria and disease was first demonstrated, it was thought that each particular species of pathogenic bacteria produced a single definite disease; and conversely, each germ disease was supposed to have its own definite species of bacterium as its cause.  Recent study has shown, however, that this is not wholly true.  It is true that some diseases do have such a definite relation to definite bacteria.  The anthrax germ, for example, will always produce anthrax, no matter where or how it is inoculated into the body.  So, also, in quite a number of other cases distinct specific bacteria are associated with distinct diseases.  But, on the other hand, there are some pathogenic bacteria which are not so definite in their action, and produce different results in accordance with circumstances, the effect varying both with the organ attacked and with the condition of the individual.  For instance, a considerable number of different types of blood poisoning, septicaemia, pyaemia, gangrene, inflammation of wounds, or formation of pus from slight skin wounds—­indeed, a host of miscellaneous troubles, ranging all the way from a slight pus formation to a violent and severe blood poisoning—­all appear to be caused by bacteria, and it is impossible to make out any definite species associated with the different types of these troubles.  There are three common forms of so-called pus cocci, and these are found almost indiscriminately with various types of inflammatory troubles.  Moreover, these species of bacteria are found with almost absolute constancy in and around the body, even in health.  They are on the clothing, on the skin, in the mouth and alimentary canal.  Here they exist, commonly doing no harm.  They have, however, the power of doing injury if by chance they get into wounds.  But their power of doing injury varies both with the condition of the individual and with variations in the bacteria themselves.  If the individual is in a good condition of health these bacteria have little power of injuring him even when they do get into such wounds, while at times of feeble vitality they may do much more injury, and take the occasion of any little cut or bruise to enter under the skin and give rise to inflammation and pus.  Some people will develop slight abscesses or slight inflammations whenever the skin is bruised, while with others such bruises or cuts heal at once without trouble.  Both are doubtless subject to the same chance of infection, but the one resists, while the other does not.  In common parlance, we say that such a tendency to abscesses indicates a bad condition of the blood—­a phrase which means nothing.  Further, we find that the same species of bacterium