Evidently the degree of infection is dependent in large measure on the relation between the _virulence_ of the invading organism and the _susceptibility_ of the host. High virulence and great susceptibility mean a severe infection; low virulence and little susceptibility a slight infection; while high virulence and little susceptibility or low virulence and great susceptibility might mean a moderate infection varying in either direction. Other factors influencing the degree of infection are the number of organisms introduced, the point where they are introduced and various conditions. These will be discussed in another connection (Chapter XXV). The study of pathogenic bacteriology includes the thorough study of the individual organisms according to the methods already given (Chapters XVIII-XXI) as an aid to diagnosis and subsequent treatment, bacteriological or other, in a given disease. Of far greater importance than the _treatment_, which in most infectious diseases is not specific, is the _prevention_ and _ultimate eradication_ of all infectious diseases. To accomplish these objects involves further a study of the _conditions under which pathogenic organisms exist outside the body_, _the paths of entrance into and elimination from the body_ and those _agencies within the body itself_ which make it _less susceptible to infection or overcome the infective agent after its introduction_. That condition of the body itself which prevents any manifestation of a virulent pathogenic organism after it has been once introduced is spoken of as _immunity_ in the modern sense. Immunity is thus the opposite of susceptibility and may exist in varying degrees. That scientists are and have been for some years in possession of sufficient knowledge to permit of the prevention and eradication of most, if not all, of our infectious diseases can scarcely be questioned. The practical application of this knowledge presents many difficulties, the chief of which is the absence of a public sufficiently enlightened to permit the expenditure of the necessary funds. Time and educative effort alone can surmount this difficulty. It will probably be years yet, but it will certainly be accomplished. CHAPTER XXIII. PATHOGENIC BACTERIA OUTSIDE THE BODY. Pathogenic bacteria may exist outside the body of the host under a variety of conditions as follows: