DISCUSSION OP INFECTIOUS DISEASES. 359 



tagion." These words, however, 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, ditfering 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 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 

 pleuropneumonia, 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 infec- 

 tious diseases, the specific bacteria of which are transmitted from 

 one animal to another, as with the contagious diseases, but the bac- 

 teria msij, under certain favorable conditions, find food enough in 

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

 tent after they have left the sick animal and before they gain entrance 

 into a healthy one. 



This general classification is subject to change if we take other 

 characteristics into consideration. Thus tuberculosis, because of its 

 insidious beginning and slow course, would not bj^ many be consid- 

 ered contagious in the sense that foot-and-mouth disease is; yet, in 

 either case, the bacillus must come from preexisting disease. The 

 disease of rabies, or hydrophobia, is not contagious in the sense that 

 rinderpest is, because the A'irus 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, 

 with positive results, into a healthy animal. 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 l)e easily under- 

 stood if the different infectious diseases in the following pages are 

 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 



