RELATION OF BACTEBIA TO DISEASE. 259 



Other low forms of plant and animal life possess similar 

 properties. 



Certain fungi are known to live as parasites on the 

 higher animals. Thus, some skin diseases of man are due 

 to this class of organisms. These may be grouped together 

 under the head of fungous diseases. 



Again, certain uni-cellular animal organisms, belonging 

 to the group of protozoa, invade the body and give rise to 

 disease. Malaria is due to one of these animal parasites. 

 The diseases due to these organisms can be designated as 

 protozoal. 



The bacterial, fungous and protozoal affections are 

 included in the large group of infectious or microbic diseases. 

 It has been stated that the cause of certain diseases, like 

 measles and scarlet fever, is as yet wholly unknown. The 

 cause, when discovered, may belong to one of these three 

 groups, but not necessarily so. It is customary to consider 

 the bacterial cell as the smallest living mass of matter. It 

 is difficult, indeed, to conceive of the existence of organized 

 beings smaller than the " infinitely small bacteria." And 

 yet, the fact that all known methods of study have failed 

 to discover the cause of certain diseases would indicate 

 that we must look to an undiscovered group of microbes 

 which must perforce be smaller than any known bacteria. 



The first step has already been taken in the direction 

 of opening up this new field of study. By means of the 

 coUodium sac method, which will be described in Chapter 

 XIV. , it has been shown that the cause of pleuro-pneumonia 

 in cattle is an organism so small, that, with the best micro- 

 scope and a magnification of 2000 diameters, it is still 

 impossible to make out its form. Evidently, this is the- 

 first of a new group of microbes. 



Pathogenic micro-organisms are of first importance in 

 so far as they produce disease in man and in the lower 

 animals. The fact, however, should not be lost sight of 

 that similar organisms produce disease in plants. A large 



