CONTAGION AND CONTAGIOUS DISEASES. 131 



essential, as in rabies, pleuro-pneumonia, and other diseases 

 where inoculation or cohabitation is necessary for the trans- 

 mission of the disease. 



Other contagious obligatory parasites, more resistant, are 

 preserved for varying periods of time outside of the animal 

 body, without, however, multiplying, and conveyed to healthy 

 animals by various means, such as water, soil, clothing, &c. — 

 contact of a healthy with a diseased animal not being necessary. 



Contagious facultative parasites are those which, as already 

 stated, live and multiply not only in the bodies of animals, but 

 also in external media, — dead organic matter, water, soil, and 

 even food. Some of these lose their virulence whilst passing 

 p through the animal body, and the disease which they induce 

 more or less quickly dies out. We can in this way account for 

 the subsidence, or, as is sometimes the case, the total disappear- 

 ance of certain epizootics, — such as epizootic catarrhal fever, 

 epizootic cellulitis, &c. — and for the sudden reappearance of 

 these diseases. Whilst the majority of these parasites seem to 

 have their virulence gradually attenuated whilst passing through 

 the animal body, they still retain the power of transmitting 

 their pathological effects, when conveyed by a diseased or newly 

 recovered animal from one part of the country to another; 

 hence it may be stated that a disease miasmatic in its origin 

 may become contagious. 



It often follows that after a severe and widely distributed 

 outbreak of some epizootic the disease disappears for a long 

 period of time ; may it not be concluded from this fact that the 

 germs have a resting stage, and that they are only called into 

 activity by some telluric or atmospheric conditions, the nature 

 of which has as yet escaped detection ? Cognisable atmospheric 

 changes have but little effect in callincr the dormant virulence 

 into activity, for we witness outbreaks of epizootic diseases in 

 all kinds of weather. Long-continued wet or damp weather 

 has sometimes an appreciable effect, but there are so many 

 exceptions to this that it cannot be said to be a determining 

 condition. 



It is already hinted that germs can be held in the air, in 

 water, in the soil — particularly in the superficial layers — foods, 

 stables, byres, sheep pens, vehicles, and utensils, as well as in 

 the various secretions and excretions of animals suffering from 



