INFECTIOUS DISEASES OF CATTLE. 373 



tagion." 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 Avhich may be expressed with the 

 help of these old terms. Infection, as laid down above, refei-s at 

 present in a comprehensive way to all microorganisms capable of 

 setting up disease in the body. Some microorganisms are trans- 

 mitted directly from one animal to another, and the diseases pro- 

 duced may be called contagious. Among these are included pleuro- 

 pneumonia, rinderpest, foot-and-mouth disease, rabies, cowpox, and 

 tuberculosis. Again, certain organisms are perhaps never trans- 

 mitted 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 in- 

 fectious 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 ex- 

 tent 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 pieexisting 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 mto 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 results. 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 



