WATER AND INSECT-BORNE DISEASES 451 



controllable. Were the public sufficiently well-informed, intelligent, 

 and careful, they should disappear within a very few years. Even 

 as it is, with the exception of the venereal diseases, cleanly people 

 rarely suffer from them. The venereal diseases are prevalent 

 mainly because our systems of religion and morality are still so 

 primitive that effective sanitation, like personal cleanliness in the 

 early ages of Christianity, is regarded as morally reprehensible. 

 Wilful poisoning with arsenic is a criminal offence. Wilful 

 poisoning with syphilis, even of an innocent wife or an ignorant 

 boy, is not a legal crime. 



738. The water-borne, the * filth ' diseases, enteric fever, cholera, 

 dysentery, epidemic diarrhoea, and other maladies which affect 

 particularly the alimentary canal, are also very controllable by 

 external sanitation. Except under special conditions they do not 

 spread rapidly, and their mode of existence outside the human 

 body is such that they are extremely open to attack. Careful 

 sanitation has almost eliminated them from many areas where 

 they were formerly prevalent. There is no apparent reason why, 

 with increasing knowledge and efficiency of preventive measures, 

 they should not be banished altogether. As in the case of the 

 contagious diseases, artificial selection is unnecessary, and artificial 

 immunity only desirable in localities where sources of infection 

 outside the human body are yet too numerous and widespread to 

 be controlled by the system of sanitation practised or practicable. 



739. In all probability insect-borne maladies, also, will be 

 banished in no very distant future by external sanitation. The 

 destruction of the intermediate hosts supplies a ready means of 

 attack. Already Europeans, owing to the conditions under which 

 they dwell, are rarely infected by plague in India, even when it is 

 raging amongst the natives. Malaria has been greatly diminished 

 in many parts of the world through the destruction of mosquitoes. 

 The same means have been effective against yellow fever in Havana. 

 Comparatively little as yet has been achieved against sleeping- 

 sickness, but the disease has not long been studied, and the life 

 history of the intermediate host, the tsetse fly, is not fully known. 

 If it be true that the malady is conveyed only by one species of 

 insect which is confined to the shady banks of streams, it may 

 prove possible to banish it more easily and effectively than has been 

 found possible in the case of any other insect-borne disease. In the 

 case of insect-borne diseases, then, selection is clearly out of place. 



740. As far as we are able to judge at present, air-borne 

 diseases will never be banished by external sanitation. They 



