232 THE RISE AND FALL OF DISEASE 



more deadly and common ; so common, in fact, that 

 it "Nvas regarded as tlie rigiit tiling to get it and 

 have done with it. just as often the planter to-day- 

 regards malaria. Clearly the victory is on our side, 

 and we must push it home. 



A very fascinating object-lesson is furnished by the 

 struggle between man and disease, and it is this. We 

 are apt to regard the ^'irus or germs of disease as a 

 dead chemical and poisonous substance, a substance 

 which having been introduced into our system will 

 have to run itself out, during which process we may 

 or may not sur\ive. The study of the tropical 

 diseases reveals to us the f^ict that the causes of the 

 diseases are organised living elements — we term them 

 parasites — which are struggling for an existence in 

 our bodies and those of animals ; like all other species 

 of lining matter, they only want to live. AVhen 

 man, however, finds out that this living is done at 

 his expense, in the shape of loss of health and very 

 often death, he bestirs himself against these competi- 

 tors. He has to adopt every means in his power to 

 ward them off, for he is now aware that these living 

 parasites in his blood or intestines are equally strug- 

 gling to survive in our bodies, and when we use one 

 method of defence they in their turn harden them- 

 selves to withstand it. This is seen, for example, in 

 the gradually increasing resistance which the parasites 

 of malaria and sleeping sickness offer respectively to 

 quinine and arsenic. A\^hen these drugs are first ad- 

 ministered they are much more efficacious, that is to 

 say, they kill more parasites ; later the parasites develop 



