INFECTIOUS DISEASES OF CATTLE. 373 



more or less from all the rest. There are, however, a few broad dis- 

 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, black quarter, 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 before they gain entrance into a healthy 

 animal. 



This general classification is subject to change if we take into consid- 

 eration other characteristics. Thus tuberculosis wo old 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 satisfactory. 

 There are so many conditions which enter into the process of infection 

 that no single classification will give a sufficiently correct or compre- 

 hensive idea, of it. These statements will be easily understood if the 

 different infectious diseases in the following pages be studied with ref- 

 erence 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 our- 

 selves 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, OH a general rule, a period of incubation 

 which comprises the time elapsing between the infection 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 seventy 

 of the fever is measured by the temperature of the animal and this is 



