EPIDEMICITY 



117 



researches solely to work in the hospital or the laboratory — for it 

 must be remembered that the parasitic causes of some diseases are 

 most probably ultramicroscopical— but to associate with the clinical 

 and experimental methods of research epidemiological researches 

 conducted by visiting several widely separated and, if possible, 

 completely detached endemic areas, in order by carefully studying 

 therein all the conditions of life and the habits of the human hosts to 

 endeavour to find factors common to the different localities. A 

 further study of these common factors from the point of view of 

 possible modes of infection may indicate one or more possibilities, 

 and then these must be put to the crucial test of experiment with a 

 view of ascertaining definitely the accuracy of the epidemiological 

 observations. 



EPIDEMICITY. 



From an endemic area diseases due to physical causes may spread 

 by the alteration, naturally or artificially, of the physical conditions 

 in the surrounding regions, and would draw back to the original 

 region upon the return to normal of these conditions. Similarly, 

 chemical causes of disease may be spread from their original re- 

 stricted area by modern methods of intercommunication- — e.g., 

 poisonous plants or their products may be brought from the tropics 

 to the Temperate Zone, and vice versa. 



In order that a parasitic disease may spread from its endemic 

 focus several factors are necessary : — 



{a) Carriers to convey it from one place to another. These may 

 be either the human host, an animal host, or the intermediary host 

 — e.g., fleas infected with plague. 



(&) In order that the disease may spread in the new area there 

 must be the suitable conditions already mentioned, 

 (c) There must also be suitable climatic conditions. 

 If these, and perhaps other still unknown factors, are present, 

 the disease will be able to spread with perhaps increased virulence, 

 first within the new area and then from one area to another, until 

 an epidemic or pandemic is produced. With the appearance of 

 paHial immunity in the human and animal hosts, altered climatic 

 and other conditions, as well as the aestivation or hibernation of 

 one of the hosts, the epidemic will die down, perhaps only to re- 

 awaken after the hibernation of the host is over or with a change of 

 climatic conditions; and this may be repeated for years, until 

 conditions become too adverse for the life of the parasite or its host, 

 when the epidemic dies away, and the disease again becomes re- 

 stricted to its endemic areas. 



The possibilities of epidemics and the presence of endemic diseases 

 raises the question of the possibility of giving man such a start in 

 life that he may be advantageously placed in the combat before him. 

 The questions involved in such a possibility constitute the large 

 and ever- increasing subject of Eugenics, to which we wiU now turn 

 our attention. 



