THE ORIGINS OF ZYMOTIC DISEASES 177 



effect a house disease which is seldom if ever contracted out- 

 side the walls of a building, tuberculosis, also, is a malady 

 of settled and more or less crowded populations. Nomadic 

 peoples never acquire it except by intercourse with settled 

 populations. Like malaria, or such contagious diseases as 

 are of long duration, it is pre-eminently capable of journey- 

 ing to the most distant parts of the globe. 



297. Zymotic disease, then, arose originally among the 

 slowly-growing populations of the Old World. Air- borne 

 diseases and diseases borne by lower animals may have arisen 

 among the early hunters and nomads. Similar forms of 

 disease, for example, distemper, rinderpest, the horse-sickness 

 in South Africa, the rabbit plague in Northern Canada and 

 the cattle-fever in Texas, occur among lower animals, when 

 these are present in considerable numbers. With the excep- 

 tion of tuberculosis and leprosy endemic disease was probably 

 almost unknown in the sparsely-peopled ancient world. The 

 facts that air- and water-borne diseases spread very rapidly, 

 that the illnesses caused by them are comparatively short 

 and sharp, and that recovery is followed by immunity must 

 have caused rapid exhaustion of the food supply of the 

 microbes. Under such conditions the persistence of the 

 pathogenetic species was maintained among the scanty popu- 

 lations by a passage to new and perhaps very distant sources 

 of supply. Introduced by travellers or spreading from tribe 

 to tribe, they appeared suddenly in epidemic form as plagues 

 and pestilences, and, disappearing as suddenly, were not 

 known again till a fresh generation furnished a fresh supply 

 of food. When, however, in spite of war, famine, and pesti- 

 lence, the human race increased to such an extent that the 

 number of fresh births furnished a perennial supply of food, 

 while at the same time a rising civilization and improved 

 means of communication lessened the isolation of various 

 communities, then many diseases slowly passed from an 

 epidemic to an endemic form. Pestilence grew less, but 

 every individual was exposed to infection, and, during youth, 

 either perished from or acquired immunity against the more 

 prevalent forms of disease. 1 



1 At the present day air-borne diseases are endemic in England. But 

 they are epidemic in places where the population is more sparse and 

 intercourse between separate communities less frequent, as in Tropical 

 Africa and Northern Asia and America. An epidemic occurs when a 

 great many members of the community are susceptible. It ceases when 

 all or most of the surviving members are immune that is after the 

 disease has attacked a very susceptible community. The disease, 

 especially if it be one which confers permanent immunity, is then apt 



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