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.
Diseases which are due to unicellular pathogenic microörganisms
are called infectious diseases, while if such diseases are transmitted under natural conditions from organism to organism they are spoken of as contagious diseases. Most infectious diseases are contagious but not all. Tetanus is a good illustration of a non-contagious infectious disease. There are very few such diseases.
When a unicellular microörganism gains entrance into the body and brings about any pathological changes there, the result is an infection. Undoubtedly many pathogenic organisms get into the body but never manifest their presence by causing disease conditions, hence do not cause an infection. It is the pathological conditions which result that constitute the infection, and not the mere invasion.
The time that elapses between the entrance of the organism and the appearance of symptoms is called the period of incubation and varies greatly in different diseases.
The term infestation is used to denote pathological conditions due to multicellular parasites. Thus an animal is infested (not infected) with tapeworms, roundworms, lice, mites, etc. Many of these conditions, probably all, are contagious, i.e., transmissable naturally from animal to animal. The word contagious has been used in a variety of ways to mean communicated by direct contact, communicated by a living something (contagium) that might be carried to a distance and finally communicable in any manner, transmissable. The agency of transmission may be very roundabout—as through a special tick in Texas fever, a mosquito in malaria, etc.,—or by direct personal contact, as generally in venereal diseases. After all, though exactness is necessary, it is better to learn all possible about the means of transmission of diseases, than quibble as to the terms to be used.