i PROTOZOA 81 



regarded by many as being the actual causal agents of the disease. 

 Regarding the details of their structure and life -history we are still 

 ignorant. 



PROTOZOA AND DISEASE 



In the course of this chapter various protozoan parasites have been 

 mentioned as the causative agents of harmful diseases of man and other 

 animals, and the student is apt to get into the habit of thinking of such 

 parasitic protozoa as being necessarily and naturally associated with 

 disease. As a matter of fact however there is reason to believe that 

 the production of disease is by no means a natural characteristic of such 

 protozoan parasites : disease is caused rather when the parasite finds 

 itself in some host other than that which is normal to it. 



When some particular species of protozoan parasite is introduced 

 into the body of a host animal this latter may behave towards the parasite 

 in any one of three different ways. 



I. It may show itself to be repellent : the intruding organism is 

 rapidly killed and destroyed. 



II. It may show itself to be tolerant towards the particular type of 

 parasite : the latter continues to live and reproduce but without increasing 

 to such an extent as to cause perceptible interference with the health of 

 the host. 



III. It may show itself to be susceptible : the parasite not only lives 

 and multiplies but it increases to such an extent as to upset to a less 

 or greater extent the normal living activities of the host in other words 

 it produces disease. 



Now under natural conditions, individuals or strains of individuals 

 which are in any degree susceptible towards such protozoa as are liable 

 to be introduced into their blood, are through this handicap being 

 constantly eliminated in the intense struggle for existence. Consequently 

 we find in Nature that the vast majority of individual animals in any 

 particular locality show little or no susceptibility towards such protozoa 

 of that neighbourhood as are likely to gain access to their bodies : they 

 are either tolerant or repellent towards them. 



It will be readily understood that the development of tolerance on 

 the part of the host will be helped by Nature acting on the parasite for 

 it is clearly to the advantage of the parasite that it should be able to live 

 within the body of the host without causing its disease or death. Strains 

 of parasites will tend to flourish and increase in proportion as they are 

 readily tolerated by the host, while strains of parasites towards which 

 a particular species of host animal is markedly susceptible will stand a 



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