What Parasites Are 313 



their life as parasites, the remainder of it being passed 

 outside, and independent of, the host's body. Others 

 exist only as parasites. Of the latter, some kinds 

 pass their whole life from generation to generation in 

 the same species of host ; others spend a part of their 

 lives as parasites in one species of host and require 

 transfer to another species to complete their life-history. 

 Some kinds, which do not ordinarily exist as parasites, 

 become parasitic on accidentally entering the body of 

 an animal. Parasites never originate spontaneously, 

 but always come from parents which are like themselves 

 at some stage in their life -history. 



Very young and very old animals are more liable to 

 attack by parasites than vigorous middle-aged ones. 

 Any conditions that lessen the vigor of animals tend 

 to increase their susceptibility to parasitic diseases. 

 Filth, crowding large numbers of animals together, and, 

 lack of sufficient food are the general predisposing 

 agencies. The opposite conditions — clean quarters, 

 clean skin, plenty of nourishing, laxative food, exercise 

 and sunlight — are invaluable in preventing these dis- 

 eases and in the successful treatment of them. 



RINGWORM 



Ringworm is a disease caused by a fungus that grows 

 in the skin. It is usually seen in cattle, but it may 

 occur in horses, dogs, cats, as well as in man himself. 

 In cattle it is most frequent toward spring, after the 

 animals have been confined in stables during the winter. 

 It is often called "barn itch." The disease is not read- 



