NATURAL RESISTANCE 



available indicates that it actively bores its way into the 

 tissues, which are apparently dissolved by ferments se- 

 creted by the ciliate. Other species of human protozoa 

 also invade the tissues of the host and are thus patho- 

 genic. There is no activity among free-living protozoa 

 exactly like this; otherwise they would be classed with 

 the parasitic species. The invasion of tissue is thus a 

 characteristic peculiar to the latter. 



Symptoms. The effects of parasitic protozoa on their 

 environment, the host, is in many cases very striking, 

 since not only are changes which constitute what we call 

 disease produced but often the death of the host results. 

 Symptoms are nothing but the results of the functional 

 modification of the organs of the host. These modifica- 

 tions in the medium in which they live are well known 

 and the changes observed in the medium correspond to 

 the symptoms resulting from parasitic activities. In other 

 words, the host is an environment just as a body of 

 water is an environment. 



Natural resistance. Each host offers certain obstacles 

 which must be overcome by the parasite before invasion 

 is accomplished ; in many cases, in fact, hosts do not be- 

 come infected at all, because of the natural resistance of 

 the body, although parasites succeed in gaining entrance 

 to the intestine or blood stream. We may compare the 

 host with a pool of water which contains various sub- 

 stances in solution and also various species of plants and 

 animals. Not all free-living protozoa succeed in popu- 

 lating such a pool of water due to the natural resistance 

 offered by the composition of the water and the other 

 organisms present — the physical, chemical and biological 



13 



