animal origin_ and the _microörganisms_ which act as injurious ferments within the animal body. These will be treated more fully later on. Of the microörganisms it may be said here, that they are almost certainly the cause of all transmissible diseases. These diseases are variously named on the basis of different ideas. They are =enthetic=, that is implanted as a seed is planted in the ground to grow and multiply. They are =zymotic= or fermentative because the essential cause multiplies and is propagated like a ferment. They are =contagious= because propagated by contact mediate or intermediate. They are =infectious= when transmitted, not alone by contact but through the atmosphere. They are =epizootic= because they tend to attack animals generally or a given genus or family of animals generally when these are exposed to the infection. They are =enzootic= when confined to the animals in a given locality, the soil or conditions of which are favorable to the preservation of the germ in pathogenic potency, or to the production of a special susceptibility in the animal system. They are =sporadic= when each case occurs without any casual relation to another. They are called =panzootic= when they attack all animals without apparent preference. The term _panzootic_ is also used to describe those recrudescences of a disease or cycles of exalted pathogenesis which are observed in contagious diseases, which frequently last for years and again give place to a period of benignancy. Such cycles, of malignancy and benignancy, may be due to modified environment acting either on the disease germ or the animal system, or on both simultaneously. The terms =enthetic=, =zymotic=, and =contagious= best express modern views of the nature of these maladies. The term =infectious= when used to express a gaseous or otherwise intangible (unorganized) body, or influence transmitted through the air, necessarily excludes the particulate, living, self-propogating germ upon which the transmissibility of the disease depends. A chemical, electrical, or other body or influence generated outside the animal body, cannot well be conceived of as reproducing itself within the animal body but must act like any other ectogenous poison, according to the size of the dose and the frequency of its exhibition. This might create an _enzootic_ disease but would lack all the qualities of a contagious affection since it could not spread from a victim when taken elsewhere and turned among animals which would prove equally susceptible if placed within the infecting area. Suppose on the other hand we apply the term _infectious_ to diseases in which the levity of the particulate living germ allows of its being inhaled into the body of the susceptible animal, the case becomes one of simple _mediate contagion_, the air acting as the intermediate bearer.