DISEASES OF CATTLE 225 



in a few days under like conditions. This matter will be referred to 

 again when we come to discuss the subject of disinfection. 



Of the protozoa which cause disease very little is at present 

 known. These parasites have a more complex life history than bac- 

 teria; and their thorough investigation is at present hampered with 

 great difficulties. 



The differences in the symptoms and lesions of the various in- 

 fectious diseases are due to differences in the respective organisms 

 causing them. 'Similarly the great differences observed in the 

 sources from which animals become infected and the manner in 

 which infection takes place are due to differences in the life history 

 of these minute organisms. Much discussion has taken place of 

 late years concerning the precise meaning of the words infection 

 and contagion. But these words are now wholly inadequate to 

 express the complex processes of infection, and it may be said that 

 each species of bacterium or protozoon has its own peculiar way of 

 invading the animal body, differing more or less from all the rest. 

 There are, however, a few broad distinctions which may be expressed 

 with the help of these old terms. Infection, as laid down above, 

 refers at present in a comprehensive way to all micro-organisms ca- 

 pable of setting up disease in the body. Some micro-organisms are 

 transmitted directly from one animal to another, and the diseases 

 produced may be called contagious. Among these are included 

 pleuro-pneumonia, rinderpest, foot-and-mouth disease, rabies, cow- 

 pox, and tuberculosis. Again, certain organisms are perhaps never 

 transmitted from one animal to another, but may come from the 

 soil. Among these are tetanus, blackleg, anthrax to a large extent, 

 and perhaps actinomycosis in part. These diseases, according to 

 some authorities, may be called miasmatic. There is a third class 

 of infectious diseases of which the specific bacteria are transmitted 

 from one animal to another, as with the contagious diseases, but the 

 bacteria may, under certain favorable conditions, find enough food 

 in the soil and the surroundings of animals to multiply to some 

 extent after they have left the sick and before they gain entrance 

 into a healthy animal. 



This general classification is subject to change if we take into 

 consideration other characteristics. Thus tuberculosis would not 

 by many be considered contagious in the sense that foot-and-mouth 

 disease is, because of the insidious beginning and slow course of 

 the disease. Yet the bacillus must come from pre-existing disease 

 in either case. The disease of rabies, or hydrophobia, is not con- 

 tagious in the sense that rinderpest is, because the virus of rabies 

 must be inoculated into a wound before it can take effect. Yet in 

 both cases the virus passes without modification from one animal to 

 another, though in different ways. 



Again, all the diseases under the second group, which seem to 

 come from the soil and from pastures, are in one sense contagious 

 in that the virus may be taken from a sick animal and inoculated 

 directly into a healthy animal with positive result. Other illustra- 

 tions may be cited which show that these old terms are not in them- 



