DISCUSSION OF INFECTIOUS DISEASES. 357 



caused by the invasion and midliplication of certain very minute 

 unicellular organisms included under the general classes of bacteria 

 and protozoa. Nearly all the diseases of cattle for which a definite 

 cause has been traced are from bacteria. Among these are tubercu- 

 losis, anthrax, blackleg, and tetanus (or lockjaw). Some diseases, 

 such as Texas fever and nagana, are traceable to protozoa, while 

 others, like actinomycosis and aspergillosis, are caused by fungi. 

 Those diseases of which the cause is unknown or imperfectly worked 

 out are pleuropneumonia, rinderpest, foot-and-mouth disease, rabies, 

 cowjDox, malignant catarrh, and dj^sentery. 



Bacteria may be defined as very minute, unicellular organisms of a 

 plantlike character. Their form is very simple, as may be seen from 

 an inspection of the various species depicted on Plate XXVIII. 

 The description of these figures will be found on page 358. The 

 magnification there given will furnish the reader some idea of their 

 very minute size. They multiply in two ways. The bacterium elon- 

 gates and then divides in the middle to form 2 daughter cells. These 

 go through the same process at once, and thus 4 cells are produced. 

 The division of these leads to 8, the division of 8 to 16, and so 

 on indefinitely. The rap>idity Avith which this multiplication takes 

 place depends upon the nature of the bacterium. The bacillus of 

 tuberculosis multiplies very slowly, while that of anthrax does so 

 with great rapidity, provided both are in the most favorable condi- 

 tion. Another mode of reproduction, limited to certain classes of 

 bacteria, consists in the formation of a spore within the body of the 

 bacterium. Spore formation usually takes place when the conditions 

 pertaining to the growth of the bacteria become unfavorable. The 

 spores are much more resistant to destructive agents than the liacteria 

 which produced them. The anthrax spore may li^•e several years in 

 a dried state, but the anthrax bacillus perishes in a few days under 

 like conditions. This matter will ))e referred to again wiien we come 

 to discuss the subject of disinfection. 



Of the protozoa which cause disease very little is at present 

 known. One which produces Texas fever is pictured on Plate 

 XLV, in figs. 4 and 5. These parasites have a more complex life 

 history than bacteria; and as they can not be grown in artificial 

 media, their thorough investigation is at present hampered with 

 great difficulties. 



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

 tious diseases are due to differences in the respective organisms caus- 

 ing them. Similarly the great differences observed in the sources 

 from which animals become infected and the manner in which infec- 

 tion 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 " con- 



