Since a bacteriological diagnosis depends most commonly on growing the organisms, it is evident that material sent for examination must _never be treated with an antiseptic or preservative_. If decomposition is to be feared the only safe procedure is to _pack the material in ice_ and forward in this way. _Tuberculous material from the parenchyma_ of internal organs may be forwarded in a preservative (not _formalin_, since this makes it very difficult to stain the bacteria) as _in this special_ case a very positive diagnosis may be made by staining alone. Even here it is better to _pack in ice_ in order that the diagnosis by staining may be confirmed by inoculating the living organisms into guinea-pigs. In the case of material _from a rabid animal_ and many protozoal diseases the rule against preservatives is not absolute, since staining is a reliable diagnostic means. Even in these cases it is often desirable to inoculate animals, hence, as before stated, it is best to make it a uniform practice to _pack material for examination in ice and use no preservatives_. PART IV. GENERAL PATHOGENIC BACTERIOLOGY. CHAPTER XXII. INTRODUCTION. Pathogenic Bacteriology treats of the unicellular microörganisms which are responsible for disease conditions, _i.e._, pathological changes in other organisms. Hence not only are bacteria considered, but also other low vegetable forms, as yeasts and molds, likewise protozoa in so far as they may be pathogenic. For this reason the term pathogenic “Microbiology” has been introduced to include all these organisms. It is largely for the reason that the methods devised for the study of bacteria have been applied to the investigation of other microörganisms that the term “bacteriology” was extended to cover the entire field. The general discussion in this chapter is intended to include, therefore, microörganisms of whatever kind pathogenic to animals. The term pathogenic as applied to an organism must be understood in a purely _relative_ sense, since there is no single organism that can cause disease in all of a certain class, but each is limited to a more or less narrow range. Some form of tuberculosis attacks nearly all vertebrates, but no other classes of animals and no plants. Lockjaw or tetanus attacks most mammals, but not any other vertebrates naturally. Typhoid fever affects human beings; hog cholera, swine, etc. This point is more fully discussed in Chapter XXIII but can not be too greatly insisted upon. “The greatest enemy to mankind is man.” Exceptions to this statement do occur and are important and must be considered in efforts to protect completely human beings from disease (tuberculosis from cattle, glanders from horses, poisoning from spoiled canned goods, anthrax from hair, hides, wool, of animals dead of the disease), but the most common human diseases are derived from other human beings directly or indirectly.