82 ZOOLOGY FOR MEDICAL STUDENTS chap. 



much poorer chance of persisting as a parasite of that particular host 

 animal. 



While there thus comes about in Nature a kind of equilibrium between 

 the host animals and the parasites of a particular region that equilibrium 

 is liable to be upset by (a) the introduction of new host animals into that 

 region, or (b) the introduction of new parasites. There are then brought 

 into contiguity with one another hosts and parasites between which 

 Nature has had no time to bring about the elimination of susceptibility 

 and there are now liable to occur violent outbreaks of disease, lasting 

 until the particular species of host animal is completely exterminated 

 or on the other hand, a condition of tolerance or repellence has been 

 gradually brought about by the weeding out of the more susceptible 

 and the survival of the less susceptible strains. 



Such disturbances of equilibrium occur doubtless frequently in 

 Nature but particularly striking examples have come about through 

 the action of man. 



The white man colonizes parts of the world infected by the parasite 

 of Malaria and he suffers greatly from the attacks of the malarial parasite 

 towards which the aboriginal negro inhabitants have become tolerant, 

 while continuing to act as carriers or reservoirs of the disease. Again 

 he introduces domesticated animals into regions in which they fall 

 victims to epidemics of Trypanosomiasis or Piroplasmosis caused by 

 parasites which spread to them from the native animals in which they 

 live without causing any obvious disease. 



Or again it may be the parasite rather than the host which is trans- 

 ported into unaccustomed regions. Such seems to have been the case 

 with sleeping sickness, which has long existed on the West Coast of 

 Africa but which, conveyed up the course of the Congo by carriers, 

 developed among the natives of Uganda into an epidemic of the greatest 

 violence as compared with the comparatively feeble outbursts on the 

 West Coast where susceptibility had been in the course of time gradually 

 diminished. 



BOOKS FOR FURTHER STUDY 



I. General Text-books 



Minchin. Introduction to the Study of the Protozoa. 

 Doflein. Lehrbuch der Protozoenkunde. 



