THE ORIGINS OF ZYMOTIC DISEASES 169 



286. With few exceptions, certainly with few if any im- 

 portant exceptions, man cannot acquire zymotic disease 

 except, directly or indirectly, through the medium of other 

 men. Travellers in mountain or Arctic solitudes, in deserts 

 or uninhabited islands, are never infected. Even malaria, 

 which was long supposed to be an emanation from the soil, 

 is not contracted, as Manson, Eoss and others have demon- 

 strated, except in the neighbourhood of other and previously 

 infected human beings. Many diseases are strictly limited 

 to man, as, for example, measles, scarlatina, and chicken-pox. 

 Even when the same species of microbe (e.g. those of 

 tuberculosis or malaria) infests both man and some other 

 species of animal, the varieties differ, and, speaking generally, 

 do not pass easily from man to the lower animals, or vice versa. 

 It follows that the microbes of man's more important diseases 

 are, directly or indirectly, dependent on him for subsistence. 

 Some of them, as those of syphilis, cannot maintain existence 

 outside the human body. Others, as those of measles, are 

 capable of maintaining existence for a limited time outside 

 the body, but seemingly cannot multiply outside it. Their 

 nutritive supply, their means of increase, lies wholly within 

 the living tissues of man. Yet others, as those of cholera 

 and enteric fever, which derive their nutritive supply from 

 the contents of his alimentary canal as well as from his living 

 tissues, are able, not only to exist in the external world, but 

 to multiply there, provided that the medium in which they 

 find themselves be contaminated with human excreta. 

 Though in a true sense saprophytic, they are in a sense as 

 true really parasitic on man. 1 



287. Since the pathogenetic micro-organisms of human 

 diseases draw their nutritive supplies from man, obviously 

 no disease can persist except among populations so 



1 That they are truly parasitic is proved by the fact that they are 

 capable of invading the living tissues, e. g. the wall of the intestines. 

 It has been said that the mass of decaying vegetation in the delta of the 

 Ganges is the natural home of cholera. But, no matter how abundant 

 the vegetation, cholera does not persist in streams on the banks of which 

 the population is sparse, as, for example, the rivers of Africa and South 

 America. It would seem, therefore, that the Ganges is the home of 

 cholera, not because it contains much vegetable matter, but because it 

 is an infected sewer, the most gigantic on earth. It is possible, of course, 

 indeed it is probable, that the micro-organisms of cholera and other 

 water-borne diseases may persist for an indefinite time in water in 

 which there is much vegetable matter in solution, but in that case it 

 appears evident that they persist as saprophytes that cannot re-acquire 

 the parasitic habit. Otherwise it is difficult to understand why all 

 the streams of the tropics are not now the homes of cholera. 



