504 Dr. C. Morley Wenyon [March 3, 



the natural protective powers of Its host, it lives a secluded exist- 

 ence and runs little risk of being exterminated by other organisms. 

 It becomes structurally modified, and more and more unsuited to life 

 in the world around it. It is no longer able to survive outside the 

 body of its host, and in order to bring about its transference to new 

 hosts one of two things happens. Special resistant forms are pro- 

 duced, if there is a possibility of external exposure on the ground or 

 in water ; or special stages are evolved which can continue their 

 development in hosts, such as blood-sucking insects, which are 

 capable of taking them up directly from the blood. The production 

 of these special forms takes time, and if they are not produced before 

 the host is overrun and killed, all the parasites in that host will 

 perish with it. Accordingly, it is found in nature that a definite 

 balance is struck between the host and its parasite. The latter does 

 as little damage as possible so long as it can multiply satisfactorily 

 and finally produce the transmission forms, while the recuperative 

 powers of the host are continuously repairing the damage done. 



By the time the special transmission forms of a parasite have 

 appeared, and there has been a sufficient interval for their transference 

 to another host, the parasite may cease to multiply, those already 

 present die off, and the infection of the host automatically comes to 

 an end. On the other hand, the damage done, though not killing 

 the host rapidly, may do so gradually, the parasite being not incon- 

 venienced by this unpleasant result so long as it has attained its 

 object in enabling its offspring to gain a footing elsewhere. 



It can be accepted generally that instances of infection with 

 parasites which bring about the death of the host in a short time 

 are unnatural ones. The trypanosomes, which cause nagana diseases- 

 of cattle and other domestic animals in Africa, produce a virulent 

 and rapidly fatal infection, but these animals cannot be regarded as 

 the natural hosts. The latter are to be sought for amongst the wild 

 game which harbour the same trypanosomes without showing, as far 

 as can be detected, any symptoms due to their presence. It is this 

 fact which makes it extremely difficult to exterminate the disease in 

 Africa, for the tse-tse fly has every opportunity of infecting itself 

 from the game and handing on the trypanosome to unnatural hosts, 

 such as the domestic animals. In the case of the parasites of malaria 

 we have another illustration of the same fact. A native who 

 has been infected with the parasites from his childhood upwards, 

 though still harbouring them, suffers comparatively little ; but imme- 

 diately unaccustomed hosts, in the shape of Europeans, become exposed 

 to the same infection, as in the case of the British armies in Mace- 

 donia, East xAfrica and elsewhere, the story is a very different one. 



Under natural conditions there is a definite balance between host 

 and parasite which holds good for all classes of infection, with few 

 exceptions. If we seek the reasons for this balance we are led into 

 the questions of immunity and tolerance and virulence of particular 



