l] CONDITIONS AFFECTING TRANSMISSION 7 



the latter serves as a reservoir for the infection and in the 

 presence of an efficient transmitting agent the parasite may 

 persist indefinitely. 



It is now known that the parasite of sleeping sickness 

 {Trypanosoma gambiense) is maintained in this manner, since 

 in addition to man it infests other vertebrate hosts, e.g. ante- 

 lopes, which are practically unaffected by it. 



In the case of the malaria parasite an equilibrium is becom- 

 ing established between it and its vertebrate host, man, but 

 only in those races that have been exposed to infection for a 

 considerable period. As a result the latter serve as reservoirs 

 of infection for those races that have not become immunized, 

 in whom the parasite produces malaria, a disease which is 

 often fatal unless checked by suitable treatment. 



The relations between the parasites and their hosts are very 

 complicated and are not yet thoroughly understood. From a 

 biological point of view, it is probable that all protozoal blood 

 parasites carried by biting-flies are primarily insect parasites 

 and have only secondarily become adapted for living in the 

 blood of vertebrates. Certainly in every case where the life- 

 cycle of the parasite is known the insect is the definitive host, 

 and the equilibrium between the parasite and its insect host 

 seems to be well established, whilst this is not the case with 

 regard to the mutual relations in the vertebrate host. 



The extreme pathogenicity of certain protozoal infections 

 in vertebrates, e.g. yellow fever in man, the various trypano- 

 somiases of man and animals, etc., strongly suggests that these 

 diseases are of comparatively recent origin and there has not 

 yet been time for any balance to become established between 

 the parasites and their respective hosts. 



The trypanosomes constitute an excellent example of a 

 group of flagellates which primitively inhabited the alimentary 

 tracts of various invertebrates, and have recently become 

 adapted to a parasitic mode of life in the blood of vertebrates. 



A large variety of insects, both biting and non-biting, 

 contain parasitic flagellates, Crithidia, Leptomonas, etc., in 

 their digestive tracts, and there are all stages between fla- 

 gellates which are just capable of living in the blood of 



