INFECTIOUS DISEASES OF CATTLE. 



359 



tinctions 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 microorganisms capable of setting up disease in the body. 

 Some microorganisms 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 dis- 

 ease, rabies, cowpox, 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, accord- 

 ing 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 con- 

 sideration 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 preexisting disease in 

 either case. The disease of rabies, or hydrophobia, is not contagious 

 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 illustrations may 

 be cited which show that these old terms are not in themselves satis- 

 factory. There are so many conditions which enter into the process 

 of infection that no single classification will give a sufficiently correct 

 or comprehensive idea of it. These statements will be easily under- 

 stood if the different infectious diseases in the following pages be 

 studied with reference to the way or ways in which each disease may 

 be contracted. Enough has been said, therefore, to show that, if we 

 wish to make ourselves acquainted with the dangers of any given 

 disease, we must study that disease and not rely upon any single word 

 to tell the whole story. 



Infectious diseases have, as a general rule, a period of incubation, 

 which comprises the time elapsing between the exposure to the infec- 

 tion and the actual appearance of the' disease. This period varies 

 with the malady. The most common symptom of this class of diseases 

 is fever. The severity of the fever is measured by the temperature 



