ENDEMICITY 



115 



we feel that a few general remarks on endemicity and epidemicity 

 as applied to disease in general may not be amiss. 



The presence of a given disease in a locality depends primarily 

 upon the presence of conditions favourable for the action of the 

 causal agent, and upon the presence of suitable vehicles for its convey- 

 ance into the human body, which must be non-immune to its attack. 

 The causal agents of disease may be physical, chemical, or parasitic. 



Physical causes of disease are often cosmopolitan in their distribu- 

 tion — as, for example, the action of gravity in producing the trau- 

 matisms brought about by falls; but though some causes, such as 

 the rays from the sun, are also cosmopolitan, still they are unable to 

 produce deleterious effects upon human beings unless assisted by 

 secondary influences, such as latitude, altitude, the rotation and 

 inclination of the world, etc., while some are restricted — as, for 

 example, the traumatisms due to ferocious animals, which, though 

 occasionally taking place in other parts, whither the animals have 

 been artificially conveyed, generally only occur in the natural 

 habitats of these creatures. 



Chemical causes of disease have become largely cosmopolitan in 

 distribution owing to improved methods of intercommunication 

 and the manufacture of chemical principles; still, certain chemical 

 causes have very restricted localities even to-day, because they are 

 little known, and only affect primitive peoples living in these regions 

 — e.g., Gloriosa superba, etc. 



The parasitic causes of disease may be animal or vegetal. The 

 endemicity of a given parasite depends upon {a) the presence of non- 

 immune human beings, in which part of the life-cycle can be gone 

 through, and which for convenience we will term the human hosts ; 



(b) easy modes of escape from the human host into the exterior; 



(c) suitable means of continuing the life-cycle in the exterior- — i.e., 

 in earth, water, air, on or in other animals or plants, intermediary 

 hosts we will call them; {d) ready means of re-entry into the human 

 host or into some other animal host in which the life-cycle is com- 

 pleted; {e) partially immune animal hosts or partially immune 

 human hosts to act as reservoirs or carriers, to enable the parasite 

 to complete its life-cycle without producing marked pathological 

 changes in the host. Examples of these may be found in the 

 malarial parasite attacking (^) the white man and the native child 

 in West Africa and elsewhere, and passing by the agency of ib) blood- 

 sucking into (c) Anopheles costalis, which, by the act of biting, 



(d) conveys the parasite to another non-immune host or to the 

 partially immune adult native, who acts as a reservoir or carrier. 



Another example is the passage of the plague bacillus from the 

 rat via the flea back to the rat or man. 



Other examples are the amoebae of dysentery, which pass from 

 man by the evacuations on to vegetal substances or into water, by 

 which they may be reintroduced into man direct or by the agency 

 of flies. 



By a suitable non-immune host is meant a host which, while 



