DISEASES OP THE BLOOD. 113 



organisms whose existence we can at present affirm only 

 from reasoning, they being probably too small to be seen 

 in the present state of optics. Many have only just been 

 discovered, and their nature and action are by no means de- 

 cidedly ascertained. Others, as the anthrax organism, we 

 are fairly well acquainted with. These diseases often assume 

 the epizootic or panzootic character, the atmosphere seems 

 to be the temporary habitat of the organisms, which occur 

 in groups, — disease clouds — much as shoal of fish in the sea. 

 Such a theory will account for many obscure facts of cause 

 and propagation of influenza and other diseases. It is well 

 known that blights occur in clouds ; why should not also 

 disease germs which are probably closely allied in nature ? 

 Certain conditions of climate are found favorable to distri- 

 bution of disease in this manner. Heat, with moisture, states 

 favorable to the growth of fungi, most readily promote 

 the spread of specific disease. Frosty weather, on the 

 other hand, is very healthy and checks prevailing epizootics. 

 Again, it has been found that currents of air, as prevailing 

 winds, may determine the direction of passage of specific 

 disorders of a certain kind, though not so as to impede 

 less rapid and energetic progress in other directions. 

 Diseases of this kind, then, seem to be communicated 

 without contact of a diseased animal, or of objects which 

 have been in contact with one, with a healthy animal of 

 the same or other species ; this is termed Infection. 

 But we can readily admit that certain disease- bearing 

 organisms cannot be taken up and diffused by the air, 

 though they may be communicated from one animal to 

 another when actual contact occurs, such is Immediate 

 Contagion ; when the contact is indirect as when the two 

 animals are fed out of the same bucket or taken care of by 

 the same man, either man or bucket may convey the morbid 

 producers from the diseased animal and thus act as the 

 vehicle, by means of which Mediate Contagion is brought. 

 Inoculation consists in the introduction of the active 

 disease producer directly into the blood-vessels or areolar 

 tissue of an animal, or causing its absorption through an 

 open wound or an exposed vascular surface. It will be 



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