What Parasites Are 315 
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 inerease 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, 
elean 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 
oeeur in horses, dogs, eats, as well as in man himself. 
In eattle it is most frequent toward spring, after the 
animals have been confined in stables during the winter. 
Tt is often called “barn itch.” The disease is not read- 
