32 SPECIFIC AND CONTAGIOUS DISEASES. 



vicinal manifestation, and the extent of the outbreak, thc}^ are 

 largely influenced — the whole class of the zymotic, contagious, 

 and specific diseases in animals — by surrounding local conditions 

 and circumstances, such operating causes constituting the so- 

 called enzootic influences, and any diseases so originating are 

 recognised as enzootic diseases. The moulding, determining, 

 or enzootic influences are such as may be peculiar to the 

 physical nature or character of the district, its geological 

 formation, its altitude, nature of soil, character of the water- 

 supply, variations of temperature, etc. ; also such influences as 

 operate through medium of food, character of the location 

 or stabling, ventilation and sanitary conditions, nature and 

 amount of the work they are engaged in, and the age of the 

 animals. 



When these same diseases so originating rapidly extend 

 from the foci of their origin and attack great numbers of 

 animals, at the same time and at varymg intervals of time, 

 they are spoken of as epizodtic diseases, or ixtnzootic diseases ; 

 and the influences determining the expansion or spread of 

 such diseases are termed epizootic or 'panzootic influences. 



The conditions which favour the passing into active opera- 

 tion of the enzootic influences are many, chief of Avhich are 

 associated with the specific morbid virus itself, and the facilities 

 offered by the living soil in which it is to be implanted. The 

 specific disease material upon which all these diseases exist, 

 and without which they would cease to be, although not always, 

 in a state of activity, is never entirely and absolutely destroyed, 

 but is ever ready, from some centre where it exists, to develop 

 and extend itself, and become epizootic when surrounding- 

 conditions are favourable. 



The entire history of these specific diseases gives us the 

 same information : the constancy of the one persisting element, 

 the infecting virus, their irregular alternations of active de- 

 velopment and recurring quietude. 



It also tells us of the undiminished power or virulence of 

 the materies morhi, its unvarying modes of development, its 

 unvarying issue in similar organic changes in the animal body, 

 and its production with no variation of the same identical 

 specific virus or contagious entity by which it has been pro- 

 pagated from age to age. 



